Five Iranian speedboats harassed three US navy ships at the weekend, approaching them and radioing a threat to blow them up, US officials say.
The incident happened in the Strait of Hormuz, a major oil shipping route. The US said their ships were about to open fire when the Iranian boats withdrew.
The White House warned Iran against "provocative actions that could lead to a dangerous incident in the future".
Iran played down the event, describing it as an "ordinary occurrence".
"This... happens for the two sides every once in a while and, after the identification of the two sides, the issue is resolved," foreign ministry spokesman Mohammad Ali Hosseini said.
Official media also reported the US statement about Iran's allegedly threatening behaviour with scepticism, implying that Washington was exaggerating the incident.
The speedboats, believed to belong to Iran's Revolutionary Guards, came within about 200m of the US vessels, Pentagon officials said.
"I am coming at you. You will explode in a couple of minutes," the Iranians said in a radio transmission, according to US officials.
The Iranian boats were operating at "distances and speeds that showed reckless, dangerous and potentially hostile intent," said Pentagon spokesman Bryan Whitman.
He said at least some of the boats were visibly armed.
US sailors assumed battle stations and the captain on one of the ships was about to order an attack when the Iranian boats turned away, dropping unidentified objects in the path of the vessel, US officials said.
The confrontation, which occurred at about 0400 local time on Sunday or late on Saturday in Washington, lasted about 20 minutes, according to the US.
The Pentagon has insisted that the three US vessels - identified as navy cruiser USS Port Royal, destroyer USS Hopper and frigate USS Ingraham - were in international waters.
The incident follows a row that erupted last March when Iranian Revolutionary Guards captured 15 British sailors and held them for nearly two weeks.
Iran said the crew had strayed into Iranian waters, a claim which Britain disputed.
The Revolutionary Guards, set up in 1979 to defend the country's Islamic system, has been designated by the US as a "proliferator of weapons of mass destruction".
The latest confrontation comes as US President George Bush is to begin a tour of the Middle East on Wednesday.
Long-standing US-Iranian tensions remain over Iran's nuclear programme, although these have been somewhat reduced since the US intelligence community released a report in late 2007 that said Iran had stopped its nuclear weapons programme in 2003.
The BBC's Paul Reynolds says the key question is whether this is a one-off incident or whether it heralds a more aggressive stance by the Iranian Revolutionary Guards.
The latter policy would be unexpected, given the lowering of tension over the nuclear issue, he says, but as the incident of the captured British naval personnel showed, tensions are always high.
There is no doubt that the US is ready to respond, our correspondent adds. ( BBC )