A US warship fired warning shots at the weekend at four Iranian Revolutionary Guard vessels that approached it at high speed in the Strait of Hormuz, US defense officials said Monday.

The shots fired by the destroyer USS Mahan on Sunday put an end to the incident, which US officials denounced as "unsafe and unprofessional."

They said the crews of the small Iranian rapid attack boats were manning their weapons as they sped toward the American ship.

"Mahan established radio communications with the IRGCN vessels and issued multiple radio and visual warnings to remain clear," a US defense official said.

The Mahan also fired flares, let off smoke and sounded the ship's siren and whistle, the official said.

"Disregarding the warnings, the IRGCN vessels continued to directly approach Mahan at a high rate of speed," the official said.

"Mahan then fired three warning shots with a crew-served 50-caliber machine gun, and the IRGCN vessels arrested their high-speed approach."

A series of similar incidents took place in 2015 and early 2016 before decreasing significantly, Davis said.

The Pentagon identified 23 incidents in 2015 and 35 in the first half of 2016, the last one in August.

The Iranian Revolutionary Guards are a paramilitary force that answers directly to the Islamic republic's supreme leader.

Their boats periodically approach US warships in international waters and the Strait of Hormuz, ignoring US radio messages and giving little indication of their intentions.

The US Navy regularly denounces such incidents.

General Joe Votel, the commander of US forces in the region, has expressed concern about the risk of military escalation triggered by rogue commanders.

In January last year, the Iranians briefly captured the crew of two small US patrol boats that strayed into Iranian waters.

The 10 US sailors were released 24 hours later.

At the time of Sunday's incident, the Mahan was with two other US navy ships, the USNS Walter S. Diehl and the amphibious assault ship USS Makin Island.

The incident comes as President Barack Obama is preparing to make way for his successor Donald Trump in less than two weeks.

The Republican president-elect has nominated as his defense secretary retired Marine Corps general James Mattis, a tough-talking former head of Central Command who has not minced words criticizing Iran.

He is set to testify at his confirmation hearing in front of the Senate Armed Forces Committee on Thursday.