The incident followed a series of "aggressive" provocations by five boats believed to belong to Iran's elite Revolutionary Guard.

US officials told CNN the boats were "attack craft" which had approached US vessels in international waters on Saturday.

Oil prices rose about 30 cents to more than $98 (£50) a barrel following the incident, with traders citing increased risk of possible disruptions to oil shipments.

Iran downplayed the incident; its foreign ministry confirmed the episode but described it as "ordinary".

An Iranian foreign ministry spokesman, Mohammed Ali Hosseini, suggested the incident was a case of misidentification.

"That is something normal that takes place every now and then for each party, and it is settled after identification of the two parties," he told the state news agency, IRNA.

The incident was "similar to past ones" that were resolved "once the two sides recognised each other".

But one US official said the Iranian boats turned away "literally at the very moment that US forces were preparing to open fire".

"It is the most serious provocation of this sort that we've seen yet," said the official, who spoke on condition of anonymity.

"Five small boats were acting in a very aggressive way, charging the ships, dropping boxes in the water in front of the ships and causing our ships to take evasive manoeuvres.

"There were no injuries but there very well could have been."

The incident occurred at about 5am local time on Sunday as a US navy cruiser, destroyer and frigate made their way into the Persian Gulf.

The shipping channel separates the Arabian peninsula with Iran and is viewed as a crucial oil supply route.

Officials said the captain of one US vessel was giving the order to shoot when the Iranian ships began turning away.

A radio transmission from one of the Iranian ships said, "I am coming at you. You will explode in a couple of minutes," a US official was reported as saying.

After the threatening radio communication, US sailors manned their ships' guns, it was claimed.

In Tehran, an Iranian foreign ministry spokesman had no immediate comment when asked about the incident.

A White House spokesman, Tony Fratto, said: "We urge the Iranians to refrain from such provocative actions that could lead to a dangerous incident in the future."

Washington has long been engaged in a standoff with Tehran over Iran's nuclear programme.

Relations between the two countries soured dramatically when an Iranian civilian airliner was mistakenly shot down by US missiles in 1988. All 290 passengers and crew were killed.

This weekend's incident occurred just days before the US president, George Bush, was due to visit the Middle East.

Only last week, Bush said that one of the aims of his trip on Wednesday was to counter Iran's ambitions in the region.

In March last year, Iran seized 15 British sailors and marines operating in the Gulf from HMS Cornwall and accused them of trespassing in Iranian territory.

The British personnel were held for almost two weeks before being freed in what the Iran president, Mahmoud Ahmadinejad, referred to as a "gift" to the British people.