The White House issued a stern warning to Tehran today after Iranian speedboats buzzed three US navy ships passing through the strategic Strait of Hormuz and reportedly threatened to blow them up.

A Pentagon official said that US forces were "literally" on the verge of firing on the Iranian boats and had moved to man their guns when the Iranians turned and sped away. No shots were fired in the incident yesterday morning in one of the world's key shipping routes for crude oil.

"We urge the Iranians to refrain from such provocative actions that could lead to a dangerous incident in the future," said Gordon Johnson, spokesman for the White House's National Security Council, in a terse statement.

Details of the incident remain unclear, but the skipper of one of the five Iranians speedboats reportedly said in a radio transmission: "I’m coming at you and you will blow up in a couple of minutes."

"It is the most serious provocation of this sort that we’ve seen yet," a Pentagon official said.

But an Iranian foreign ministry spokesman dismissed it is a routine incident. "This is an ordinary occurrence which happens every now and then for both sides," he told the IRNA news agency.

News of the stand-off emerged as President Bush prepares to leave tomorrow on a trip to the Middle East in which he is expected to tackle Iran's growing regional influence.

The face-off was the most serious such incident since the Revolutionary Guard's seizure of eight Royal Navy sailors and seven Marines from the HMS Cornwall last March in the Shatt al-Arab waterway in the Gulf. Iran claimed that the 15 had strayed into Iranian waters, which Britain denied.

The incident occurred at about 5 am local time on Sunday as a US Navy cruiser, destroyer and frigate were crossing the strait on their way into the Gulf when they were buzzed by the speedboats, which dropped boxes in the water in front of them.

Bryan Whitman, a Pentagon spokesman, said that the American vessels were in international waters, making a normal transit into the Gulf. He said that the Iranian boats were operating at "distances and speeds that showed reckless and dangerous intent – reckless, dangerous and potentially hostile intent".

Mr Whitman said that the episode lasted 15 to 20 minutes but would not say whether officials know for certain whether the were vessels were Iranian Revolutionary Guard or regular Iranian navy. The Revolutionary Guard forces have been known to be more aggressive than the regular navy.

"At least some were visibly armed. Small Iranian fastboats made some aggressive manoeuvres against our vessels and indicated some hostile intent," he said.

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