A US Navy destroyer has fired multiple warning shots at Iranian patrol boats as they sped toward it at the entrance to the Persian Gulf, according to defence officials.
The crew of the USS Mahan fired the warning shots after attempting to establish contact with the Iranians and after dropping smoke flares, the officials said.
The US Navy occasionally has confrontations with Iranian naval forces in the Persian Gulf, but they do not usually reach the point of prompting warning shots.
The US officials said the Mahan was in the Strait of Hormuz on Sunday when the Iranian boats sped toward it and failed to halt despite "cautionary moves".
There were conflicting initial reports on whether the Americans and Iranians had established radio communication before the warning shots were fired.
One official said the main concern aboard the Mahan was the speed with which the Iranian boats were approaching, rather than their proximity.
This official said the boats were an estimated 900 yards away when the warning shots were fired.
The Iranian boats broke away after the warning shots were fired, and then made radio contact with the Mahan by asking its course and speed.
Further details were not immediately available.
US relations with Iran are among the tougher foreign policy issues that President-elect Donald Trump will inherit next week when he succeeds President Barack Obama.
Cover image a file photo of USS Mahan.