WASHINGTON (AP) — A U.S. destroyer fired multiple shots at Iranian patrol boats as they sped toward the destroyer at the entrance to the Persian Gulf, two U.S. defense officials said.

The crew of the USS Mahan fired the shots after attempting to establish contact with the Iranians and after dropping smoke flares, the officials said. The official was not authorized to discuss the matter publicly as so spoke on condition of anonymity.

The U.S. occasionally has confrontations with Iranian naval forces in the Persian Gulf but they do not usually reach the point of prompting shots by the U.S.

The U.S. officials said the Mahan was transiting the Strait of Hormuz on Sunday when the Iranian boats sped toward it and failed to halt despite U.S. cautionary moves. There were conflicting initial reports on whether the Americans and Iranians had established radio communication before the 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 shots were fired.

The Iranian boats broke away after the shots were fired, and then made radio contact with the Mahan by asking its course and speed.

Further details were not immediately available.

U.S. relations with are among the tougher foreign policy issues that President-elect Donald Trump will inherit next week when he succeeds President Barack Obama.