The US navy says it has been harassed and provoked by Iranian ships in the Strait of Hormuz in an incident the White House called a "serious provocation".
Officials from the Pentagon said on Monday that Iranian speedboats surrounded three US navy ships over the weekend, radioing a threat to blow them up.
"We urge the Iranians to refrain from such provocative actions that could lead to a dangerous incident in the future," Gordon Johndroe, a spokesman for the US national security council, said in a statement.
The incident occurred at about 5am local time on Sunday as a US navy cruiser, destroyer and frigate were on their way into the Gulf and passing through the strait - a major oil-shipping route.
Five small boats began advancing on the ships, dropping boxes in the water in front of the ships and forcing the US ships to take evasive manoeuvres, the Pentagon official said.
Verge of firing
There were no injuries but the US official said there could have been, because the Iranian boats turned away "literally at the very moment that US forces were preparing to open fire" in self-defence.
Iran has confirmed the incident, with the foreign ministry saying that a confrontation was "something normal" and was resolved.
Mohammed Ali Hosseini, an Iranian foreign ministry spokesman, suggested the Iranian boats had not recognised the US vessels.
But he played down the incident, suggesting it was an issue of mis-identification. He did not comment on the US claims of the Iranian boats' actions.
"That is something normal that takes place every now and then for each party, and it (the problem) is settled after identification of the two parties."
Alireza Ronaghi, Al Jazeera's correspondent in Tehran, said the commander of the country's Revolutionary Guards will appear before parliament on Tuesday to justify the measures taken by his forces to defend his country’s territorial waters.
Ronaghi said that as this would be a closed hearing, the reported incident with the US ships may be discussed.
The incident comes just days before George Bush arrives in the Middle East on a trip that some Iranian officials have said is designed to sabotage Tehran's good relations with countries in the region.
But Ronaghi said it was unlikely anyone in the Iranian administration or the armed forces would actively provoke such an incident with ships that were not in Iran's territorial waters.
In March last year, Iran seized 15 British sailors and marines in the Gulf and accused them of trespassing in Iranian territory while they inspected a merchant vessel. London maintained the British personnel were in Iraqi waters.
The British personnel were held for almost two weeks before being freed.
Tensions between the two nations have increased in recent years over Washington's allegation that Tehran has been developing nuclear weapons and supplying and training Iraqi groups using roadside bombs against US troops.
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