Five Iranian boats made aggressive manoeuvres and showed hostile intent against three US Navy ships at the weekend in the Strait of Hormuz, a major oil shipping route in the Gulf, the Pentagon said yesterday.
The Pentagon said the incident was serious. It described the Iranian actions as "careless, reckless and potentially hostile" and said Tehran should provide an explanation.
In Tehran, the Iranian Foreign Ministry described the incident as ordinary.
"The example that happened on Saturday was similar to previous cases and is an ordinary and natural issue," Foreign Ministry spokesman Mohammad Ali Hosseini told Iran's official Irna news agency.
The incident was the latest sign of tension between Washington and Tehran, at odds over a range of issues from Iran's nuclear programme to US allegations of Iranian support for terrorism.
US President George W. Bush is due to travel to the Middle East this week on a trip he has said is partly aimed at countering Iranian influence.
"We urge the Iranians to refrain from such provocative actions that could lead to a dangerous incident in the future," White House spokesman Gordon Johndroe said.
Pentagon spokesman Bryan Whitman said the three US ships had been in international waters passing through the Strait of Hormuz on Sunday when they were approached by five fast boats, at least some of which were visibly armed.
The boats were identified as Iranian and "made some aggressive manoeuvres against our vessels and indicated some hostile intent," Mr Whitman said.
"This required our vessels to issue warnings and conduct some evasive manoeuvreing. The US Navy vessels were prepared to take appropriate actions... but there was no engagement of the vessels."
Other Pentagon officials, speaking on condition of anonymity, said the Iranians made threats by radio and a US captain was in the process of ordering sailors to open fire when the Iranian boats moved away.
The incident took place about 4 a.m. GMT on Sunday, or late Saturday night in Washington, the officials said.
According to the officials, the radio transmission from one of the Iranian ships said: "I am coming at you. You will explode in a couple of minutes." The officials said it was not unusual for Iranian boats to get close to US ships in the strait but the radio transmission was unusual.
The officials said the Iranian vessels also dropped small white boxes into the water. It was not clear what the boxes contained. One official said the move may have been an attempt to ascertain what tactics the US ships would use if objects were dropped into the strait.
The Iranian boats were believed to belong to Iran's Revolutionary Guard, the officials said.
In October, the US designated the Revolutionary Guard Corps a proliferator of weapons of mass destruction and its elite Qods force a supporter of terrorism.
In March, 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 in what Iranian President Mahmoud Ahmadinejad said was a "gift" to the British people.