Pentagon spokesman Bryan Whitman said the three U.S. 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.
"This is a serious incident," Whitman said. "Clearly this is something that deserves an explanation."
"AGGRESSIVE MANOEUVRES"
The boats were identified as Iranian and "made some aggressive manoeuvres against our vessels and indicated some hostile intent", Whitman said.
"This required our vessels to issue warnings and conduct some evasive manoeuvring," Whitman told reporters. "The U.S. 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 U.S. 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. British time 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 Iranian boats were believed to belong to Iran's Revolutionary Guard, the officials said.
In October, the United States designated the Revolutionary Guard Corps a proliferator of weapons of mass destruction and its elite Qods force a supporter of terrorism.
An "informed source" from the naval force of the Revolutionary Guards was quoted by Iranian state television as saying: "There were no out of the ordinary contacts between the Guards' naval force and American ships."
In his account of the incident, the source said three U.S. naval ships were asked by Guards' vessels "as usual" to identify themselves "which they did and they continued their path".
