Iran's military warned off a US reconnaissance aircraft trying to approach Iranian naval manoeuvres, the semi-official Fars News Agency said on Tuesday.
The incident involving the two old adversaries happened on Monday, it quoted the armed forces chief as saying.
Iran's navy last week launched eight days of exercises in the Gulf and the Gulf of Oman, a region crucial for global oil supplies.
"A US reconnaissance aircraft which had intended to approach our operational war games left upon the timely warning of our air defence forces," Fars quoted armed forces commander Ataollah Salehi as saying.
He was speaking to reporters as the military test-fired two surface-to-sea missiles in the Gulf of Oman, it added.
There was no immediate US comment on the report.
Pieter Wezeman, a researcher on military issues at the Stockholm International Peace Research Institute (SIPRI), said similar incidents had happened before and did not necessarily signal an escalation in tension.
"To me it sounds like standard behaviour from both sides," he said by telephone from the Swedish capital. "I recall a number of them over the years."
Earlier this month, Iran revealed that one of its military planes had photographed a US aircraft carrier, suggesting that the US ship's crew had objected to the Iranian action.
US Defence Secretary Robert Gates said last week Iran was challenging US naval power in the Middle East with an array of offensive and defensive weapons.
The US military is present in most Gulf Arab countries and has expanded land- and sea-based missile defence systems in and around the Gulf during a protracted nuclear stand-off with Iran.
Iran's manoeuvres coincide with rising tension between Iran and the West over Tehran's uranium enrichment programme. Western officials suspect it is aimed at developing nuclear weapons capacity. Iran says it is only for electricity generation.
The United States is lobbying for a fourth round of UN sanctions on the Islamic state over its refusal to halt sensitive nuclear activities and open up to UN inspections.
Iran often announces advances in its military capabilities and tests weaponry in an apparent attempt to show its readiness for any strikes by Israel or the United States.
Israel, widely believed to have the Middle East's only nuclear arsenal, has described Iran's nuclear programme as a threat to its existence and has not ruled out military action.
In exercises held in the Gulf and the Strait of Hormuz last month, official media said the elite Revolutionary Guards tested missiles and a new speedboat capable of destroying enemy ships.
Iran, a predominantly Shi'ite Muslim state, has said it would respond to any attack by targeting US interests in the region and Israel, as well as closing the Strait of Hormuz.
About 40 percent of the world's traded oil leaves the Gulf region through the strategic narrows.
Salehi said: "It's past the epoch when America would change the regime in a country by just dispatching a warship."
Iran is "very serious about the protection of its interests", the armed forces chief added.