By Idrees Ali

WASHINGTON (Reuters) - A U.S. Navy destroyer fired three warning shots at four Iranian fast-attack vessels near the Strait of Hormuz after they closed in at high speed and disregarded repeated requests to slow down, U.S. officials said on Monday.

The incident, which occurred on Sunday and was first reported by Reuters, comes as U.S. President-elect Donald Trump prepares to take office on Jan. 20. In September, Trump vowed that any Iranian vessels that harassed the U.S. Navy in the Gulf would be "shot out of the water."

"This was an unsafe and unprofessional interaction, and that is due to the fact that they were approaching at a high level of speed with weapons manned and disregarding repeated warnings," Captain Jeff Davis, a Pentagon spokesman, said in a briefing.

The warning shots were fired at the Islamic Revolutionary Guard Corps boats by the USS Mahan after it established radio contact but failed to get them to slow down, Davis said, confirming earlier accounts of U.S. defense officials.

Those officials, speaking to Reuters on condition of anonymity, said the Iranian vessels came within 900 yards (800 meters) of the Navy destroyer, which was escorting two other U.S. military ships.

The Mahan also fired flares and a U.S. Navy helicopter also dropped a smoke float before the warning shots, they said.

The IRGC and Trump transition team were not immediately available for comment.

The White House said the Iranian vessels had behaved in an unacceptable manner and that such incidents risked increasing tensions.

"At this point the United States does not know what the intentions of the Iranian vessels were, but the behavior is not acceptable given that the USS Mahan was operating in international waters," White House spokesman Josh Earnest said during a press conference on Monday.

Years of mutual animosity eased when Washington lifted sanctions on Tehran last year after a deal to curb Iran's nuclear ambitions. But serious differences still remain over Iran's ballistic missile program as well as conflicts in Syria and Iraq.

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One of the U.S. defense officials said similar incidents occur occasionally.

Most recently in August, another U.S. Navy ship fired warning shots toward an Iranian fast-attack craft that approached two U.S. ships.

In January 2016, Iran freed 10 U.S. sailors after briefly detaining them in the Gulf.

The one official added that the warning shots fired on Sunday were just one of seven interactions the Mahan had with Iranian vessels over the weekend, but the others were judged to be safe.

(Reporting by Idrees Ali; Editing by Mohammad Zargham and Paul Simao)