"I'm coming at you and you will blow up in a couple of minutes," a Defense Department official quoted the radio transmission as saying.

Crew aboard two of the five speedboats also dumped floating boxes into the path of one of the vessels during the incident Sunday morning, but it passed them without incident, officials said.

The US government accused Iran of engaging in provocative and potentially hostile behavior and vowed "to confront Iranian behavior where it seeks to do harm to either us or our friends and allies in the region."

US Defense Secretary Robert Gates said the incident was "quite troubling actually and a matter of real concern."

"This is a very volatile area and the risk of an incident and of an incident escalating is real. I can't imagine what was on their minds," he told reporters in San Diego, California.

But Iran played down the encounter, which came just days before Bush travels to the region to boost the Israeli-Palestinian peace process and assure allies that Washington continues to view Iran as a threat.

Iranian Foreign Ministry spokesman Mohammed Ali Hosseini called it "an ordinary occurrence which happens every now and then for both sides."

Gates said there had been two or three such incidents -- "maybe not quite as dramatic as this one" -- over the past year.

Pentagon officials said the Iranian speedboats swarmed the US ships in international waters in the strait on Sunday at around 0400 GMT, peeling off into two groups as they approached.

The cruiser USS Port Royal, the destroyer USS Hopper, and the frigate USS Ingraham received a radio call threatening to blow them up and saw two of the speedboats drop floating boxes into the path of the Ingraham, officials said.

"They were called on radio, ship's whistles were sounded -- those sorts of things -- to draw attention to the fact that their maneuvers were a cause of concern to the commanding officers," said Vice Admiral Kevin Cosgriff, commander of the US Fifth Fleet.

He said the warnings went unheeded, and that the speedboats came within less than 500 meters (yards) of the US boats on a couple of occasions. Other defense officials put the distance at a couple of hundred meters.

Ship commanders responded by increasing their vessels' "onboard readiness," but no shots were fired, the admiral said. "In this case, the commanding officers did not believe they needed to fire warning shots."

Cosgriff said the incident lasted less than 30 minutes, ending when the speedboats turned and headed back to Iranian waters.

He said the US ships were at least 15 miles from the nearest Iranian land, and thus outside its 12 mile territorial limit in international waters.

The US ships were clearly marked, it was daylight and there was decent visibility, he said.

The use of armed speedboats and aggressive maneuvering fit the operating profile of Iran's Revolutionary Guard Corps, rather than the regular Iranian navy.

Cosgriff said US Navy ships frequently encounter Iranian regular navy and Republican Guard vessels in the Gulf.

"This is more serious because of the aggregate of the actions, the coordinated movement of the ships of the boats, the threats of maneuvering, the more or less simultaneous radio communication, the dropping of objects," he told reporters via video link from his headquarters in Bahrain.

"These are in my mind unnecessarily provocative."

US National Security Council spokesman Gordon Johndroe issued a statement urging Iran "to refrain from such provocative actions that could lead to a dangerous incident in the future."

State Department spokesman Sean McCormack said there was wide support in the region for Washington to confront negative Iranian behavior in hopes of encouraging Tehran to play a more constructive role there.

The Strait of Hormuz is a crucial choke point for world energy supplies, with about 20-25 percent of the world's crude oil supplies passing through from Gulf oil producers.

The US Fifth Fleet is based in Bahrain and US Navy officials say about three dozen US and coalition warships are in the region at any one time. The aircraft carrier USS Harry Truman currently is in the Gulf.

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