The United States vowed on Monday to keep operating in international waters after it said five Chinese ships veered dangerously close to a US Navy surveillance vessel in the South China Sea.
The United States formally protested to Chinese authorities in Beijing and to the defense attache in Washington over the incident, which the Pentagon said occurred on Sunday in the South China Sea, about 75 miles (120 kilometers) south of Hainan Island.
"This was a reckless, dangerous maneuver that was unprofessional" and violated international law, Defense Department spokesman Bryan Whitman told reporters on Monday.
He said one of the Chinese ships came within 25 feet (7.5 meters) of the USNS Impeccable, a surveillance vessel designed to support anti-submarine warfare, and that the Chinese crew tried to snag the cables that tow the ship's underwater sonars.
The White House demanded China respect international maritime law and said it would keep up naval operations in the area.
"We're going to continue to operate in those international waters and we expect the Chinese to observe international law around there," spokesman Robert Gibbs said.
Beijing has long expressed dissatisfaction with US surveillance operations by having Chinese ships and planes approach US naval ships in what it considers an "economic exclusion zone."
But Sunday's incident was more aggressive than previous encounters, US officials said.
A Republican lawmaker called the standoff a critical "early test" for President Barack Obama just weeks before he meets Chinese President Hu Jintao in April.
The Chinese ships surrounded the USNS Impeccable, and after two ships came within 50 feet (15 meters), waved Chinese flags and told the Americans to leave the area, the Defense Department said in a statement.
The tension on the high seas took an odd turn at one point when the Americans sprayed water at the Chinese boats and the crew responded by stripping to their underwear.
"Because the vessels' intentions were not known, Impeccable sprayed its fire hoses at one of the vessels in order to protect itself.
"The Chinese crewmembers disrobed to their underwear and continued closing to within 25 feet," said the Pentagon.
Two Chinese boats moved directly in front of the Impeccable, forcing it to take emergency action to avoid collision, and then dropped pieces of wood into its path, it said.
The Chinese vessels included a navy intelligence ship, a government fisheries patrol vessel, a state oceanographic patrol boat, and two small trawlers, it said.
The Impeccable, contracted out to the Navy with a civilian crew, was carrying out a routine mission, Whitman said.
The incident followed "increasingly aggressive conduct by Chinese vessels" in the past week -- with Chinese boats steaming near US ships and aircraft flying low overhead, the Pentagon's statement said.
The Chinese were testing the new US president and other countries would closely follow his response, a US Republican lawmaker said.
"This story will reverberate around the world and will be carefully watched in North Korea, Iran and Syria," said Republican Representative Mark Kirk, a US Navy reservist.
He urged the Obama administration to respond forcefully, perhaps by sending the surveillance ship "right back into the same area" -- this time with a destroyer escort, said Kirk.
The standoff came just weeks before a summit in April between Obama and Chinese President Hu Jintao in London.
The Pentagon recounted several incidents over the past week in which it said Chinese ships and aircraft approached at close distance without warning.
On April 1, 2001, US-China relations were seriously strained after a mid-air collision between a US surveillance aircraft and a Chinese fighter jet.
The collision killed the pilot of the Chinese jet and the US EP-3 spy plane was forced to make an emergency landing at Hainan Island. The 24-member US crew were held for 11 days in a crisis that was portrayed as a test of a new US president, George W Bush.
The United States keeps a watchful eye on China's military power, including its growing fleet of submarines.
In 2006, a Chinese submarine avoided detection and came within a few miles of the US aircraft carrier Kitty Hawk off the coast of Okinawa, Japan. The embarrassing incident prompted a US review of anti-submarine defenses.