How the US Navy is blockading Iran
The US Navy has boarded and seized two vessels, and directed 27 others to turn around or return to an Iranian port since it began a blockade against ships entering or exiting Iranian ports and coastal areas.
The blockade, in place since 10 am ET (7.30pm IST) on April 13 after the failure of talks to end the Iran war, is being enforced by at least 15 US warships. Iran has called the blockade illegal, a violation of the ceasefire between the two countries, and seizure of its container vessel an act of piracy. US allies barring Israel have refused to back the blockade.
Admiral Brad Cooper, commander of the US Central Command, said, “US forces have completely halted economic trade going in and out of Iran by sea." Iran has warned it will retaliate against the ports of US allies in the Gulf if its own ports are threatened.
What the US has done so far
The US Navy has boarded two vessels — one off Chabahar Port in the Gulf of Oman and one in the Indian Ocean — and directed 27 others to turn around or return to an Iranian port as on April 21, since it began a blockade against ships entering or exiting Iranian ports and coastal areas.
These videos released by the US military shows the seizure of Iranian-flagged cargo vessel Touska on April 20. Guided-missile destroyer USS Spruance (DDG 111) intercepted Touska as it transited the north Arabian Sea en route to Bandar Abbas. US forces issued multiple warnings and informed the Iranian-flagged vessel it was in violation of the US blockade, according to the Pentagon.
After Touska’s crew failed to comply with repeated warnings over a six-hour period, Spruance directed the vessel to evacuate its engine room. Spruance disabled Touska’s propulsion by firing several rounds from the destroyer’s 5-inch MK 45 Gun into Touska’s engine room. US Marines from the 31st Marine Expeditionary Unit later boarded the non-compliant vessel, which remains in US custody.
April 14
A US Navy destroyer interdicted two oil tankers attempting to leave Iran on Tuesday, a day after the blockade went into effect, and instructed them to turn around. The ships had left Iran’s Chabahar port on the Gulf of Oman and were contacted by the US warship via radio communication, an official told Reuters on condition of anonymity.
April 15
The USS Spruance (pictured), an Aegis guided-missile destroyer, forced an Iranian-flagged cargo vessel that tried to evade the US blockade after leaving Bandar Abbas, exiting the Strait of Hormuz, and transiting along the Iranian coastline, to turn back to Iran.
April 20
US forces took custody of Iranian-flagged cargo vessel Touska (pictured) after it attempted to breach the blockade, off the coast of Chabahar port in the Gulf of Oman. The 900-foot-long cargo ship is said to have what US deems dual-use items that could be used by the military onboard.
April 21
US forces interdicted and boarded the Tifani, a Botswana-flagged vessel, in the Indian Ocean in an area under the US Indo-Pacific Command. The operation effectively extended the US blockade to waters far away from Iran. 'International waters are not a refuge for sanctioned vessels,' the Pentagon said. The vessel had loaded two million barrels of crude on Kharg Island on April 5 and passed through the Strait of Hormuz on April 9, and was heading towards Singapore.
15 warships implementing the blockade
The US has deployed more than 15 warships to stop vessels trying to enter or leave Iranian ports, the Wall Street Journal has reported. These warships can launch troops and helicopters to go take control of a ship. They can also drive up alongside vessels and force them to go wherever they want.
These warships are operating outside the Persian Gulf to avoid the risk of going near Iran’s coastline, where they will be vulnerable to attack from drones and missiles. According to the US Department of War, the warships are positioned in the Gulf of Oman and in the Arabian Sea region.
Enforcement authority is not confined to waters near Iran — vessels could be boarded far from the Persian Gulf — even in distant international waters such as the Indian Ocean or Pacific Ocean.
A precedent for this approach emerged during the US naval blockade of sanctioned vessels trading with Venezuela, launched in December 2025. In that campaign, several tankers attempted to evade enforcement by fleeing the Caribbean, only to be tracked across oceans and seized thousands of kilometres away. US forces boarded multiple Venezuela-linked oil tankers in the Indian Ocean after pursuing them from Venezuelan waters, demonstrating that sanctions enforcement could extend well beyond the immediate blockade zone. A look at those operations offer clues as to how the US can implement its blockade against Iran.
The USS Abraham Lincoln aircraft carrier, which is in the Arabian Sea, has a large flight deck that can be used to conduct aerial boardings, similar to how the USS Gerald R Ford aircraft carrier was used to launch helicopters to seize tankers near Venezuela.
The USS Tripoli amphibious assault ship is also in the Arabian Sea, and aboard it is a marine unit that is trained to intercept ships. Tripoli also has a large flight deck so it can also deploy teams on helicopters to board hostile vessels, or send out Marine teams on smaller boats.
There are also several guided missile destroyers in the region that have small flight decks that can be used to launch helicopters or boats. The missile destroyers could also be used to sail up to a hostile vessel and marshal them to a specific area and hold them in place.
The US also has other warships and forces in the area, including submarines, details of which have been kept under wraps by the US military for security reasons.
Standard Operating Procedure and legality
Under international maritime law, specifically the United Nations Convention on the Law of the Sea (UNCLOS), any warship can approach a commercial vessel and, under ‘reasonable circumstances’, conduct a ‘right of visit’ boarding to verify nationality, check for illicit activity (piracy, slavery, unauthorised broadcasting) or enforce blockades during wartime. However, this can only be done on the high seas, or international waters.
In the current US blockade of Iranian ports, if any commercial vessel resists or refuses to comply with US orders, then the US warships can send highly trained Marines or special operations forces such as Navy SEALs to conduct a ‘hostile boarding’.
According to the Commander's Handbook on the Law of Naval Operations, a naval blockade generally follows these steps:
Warning issued: The warship identifies itself, contacts the vessel and orders it to stop.
Ship is boarded: The warship sends a boat with one officer or more to the ship for a visit and search. The officers and boat crew are armed at the discretion of the warship’s commander. If a search at sea isn’t practical, the ship is directed to a neutral area.
Checking papers: Crew are questioned about cargo, route, flag and voyage details.
Search and seizure: If ‘reasonable grounds’ exist the ship is violating the blockade, it may be captured. If it resists capture, it may be attacked.
Ship is detained, rerouted or released: The ship may be diverted, detained, captured or released depending on what’s found and the applicable authority.
If the US starts taking control of tankers, it may need a captain and crew to drive the vessels if the current crew doesn’t comply. The US would also need a place to park the tankers.
Will Iranian vessels be protected?
Iran’s ‘shadow fleet’ of oil tankers that secretly export oil in defiance of sanctions, will likely not attempt to test the American blockade but Iranian-flagged ships are more likely to attempt to transit the waterway, according to retired US naval officers. Those vessels may be guarded by Islamic Revolutionary Guard Corps troops.
Iran has warned that if its ports are threatened, then no ports in the Persian Gulf or Gulf of Oman would remain secure. Ebrahim Zolfaghari, an Iranian military spokesperson, said any blockade of vessels in international waters would amount to ‘piracy’.
How did the blockade plan emerge?
A plan to blockade the Strait of Hormuz has existed for years at US Central Command, according to the Wall Street Journal. The US Navy has previously carried out similar operations, including intercepting Iranian weapons shipments to Yemen’s Houthi rebels, retired General Frank McKenzie, a former CENTCOM commander, told the Wall Street Journal. Most recently, a blockade and quarantine was conducted against oil tankers entering or leaving Venezuela.
Military planners refined the proposal as an option during the current crisis. CENTCOM chief Admiral Brad Cooper presented it to War Secretary Pete Hegseth months ago, and President Donald Trump approved the blockade after ceasefire talks failed last week, a senior official told the Wall Street Journal.
Why it wasn’t done earlier
The operation is complex and risky. The narrow strait lies close to Iran’s coastline, exposing US ships to mines, drones and fast attack boats.
Before the conflict escalated, US officials avoided a blockade because Iran could retaliate by seizing tankers linked to US partners or mining the strait, potentially triggering a sharp rise in global oil prices. Those risks still remain.
Where are the Iranian ports?
Iran has 13 ports in the Persian Gulf along its 2,800km coastline — the longest of any country in the area. The long coastline and ports both east and west of the Strait of Hormuz give Iran the ability to exert strong control over the length of the waterway. Here are Iran's 13 ports along with some major ports of other countries in the Persian Gulf.
West of Strait of Hormuz
This map shows the major ports of all countries in the Persian Gulf that export petrochemicals and other minerals.
East of Strait of Hormuz
This map shows the major ports of all countries in the Persian Gulf that export petrochemicals and other minerals.
Where US warships are positioned
US forces are positioned in the Gulf of Oman and the Arabian Sea. They include thousands of US service members, sailors and Marines from the USS Abraham Lincoln Carrier Strike Group. However, the US said that it can seize sanctioned vessels anywhere in international waters. On April 21, the US military boarded a sanctioned tanker in the Indian Ocean, far away from where it has put its blockade in place.
26 Iranian tankers have breached blockade
While the US military says it has turned around 27 vessels and ‘completely halted’ Iranian trade, at least 26 Iranian vessels linked to the so-called 'shadow fleet' — including oil and gas tankers— have managed to breach the American blockade, according to Lloyd’s List Intelligence.
Tankers stalled near the Strait of Hormuz
Hundreds of commercial vessels carrying oil, gas, minerals and commercial goods in the Persian Gulf awaiting entry into the Strait of Hormuz. The following snapshot taken from Marine Traffic shows vessels in the area on April 21.
Waiting to transit Hormuz
The Strait of Hormuz is 167km long. Its width varies, and at its narrowest point provides 2-mile channels for inbound and outbound shipping, separated by a 2-mile buffer zone.
How does a naval blockade work?
A blockade has to be announced and its boundaries defined. This allows neutral shipping vessels to take alternative routes or request passage on humanitarian grounds. US forces will be monitoring an area about 54,389.75 square kilometres in size along the Iranian coastline, according to a calculation by USA TODAY based on a 12 nautical mile stretch of territorial water.
How Iran could retaliate
Although Iran’s military capabilities have been weakened, it still retains significant firepower, including thousands of ballistic missiles and fast-attack boats operated by the Islamic Revolutionary Guard Corps. Any attack on US ships enforcing the blockade — or clashes aboard seized tankers — could quickly escalate the conflict.
Tehran has threatened to strike ports in the Persian Gulf and Gulf of Oman in response to the blockade. Iran also owns a network of strategic islands through which it protects its ports and exerts control over the Strait of Hormuz.
The US naval blockade appears to be Washington’s fallback option after it abandoned a far riskier military plan to put boots on the ground. That plan was abandoned because regional geography heavily favours Iran, according to retired US military officials and experts.
According to multiple media and defence analyses, US planners initially examined ground assaults or surgical raids aimed at seizing Iran’s enriched nuclear material and capturing key islands that allow Tehran to dominate the Strait of Hormuz — the world’s most critical oil chokepoint.
That option was shelved — so far — as military assessments concluded that Iran’s defensive advantages — rooted in terrain, proximity and layered coastal defences — could turn any assault into a prolonged and costly war.
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