Reuters reported that the vessels had been approaching at high speed and disregarded repeated requests to slow down, citing US officials a day after the incident.
A defense official confirmed to USNI News on Monday, that the Mahan was transiting the Strait of Hormuz into the Persian Gulf when the four Iranian Revolutionary Guard Corps Navy fast inshore attack craft (FIAC) came at the destroyer at a high rate of speed with their crew-served weapons manned.
In September, president-elect Donald Trump vowed to “shoot out of the water” any Iranian vessels that harassed the US Navy in the Gulf.
“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 by the USS Mahan after it established radio contact but failed to get them to slow down, Davis said, confirming earlier accounts by US defense officials.
The officials, speaking to Reuters on condition of anonymity, said the Iranian vessels came within 800 meters of the Navy destroyer, which was escorting two other US military ships.
The Mahan also fired flares and a US Navy helicopter dropped a smoke float before the warning shots, they said.
The Trump transition team were not immediately available for comment.
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 present immediate danger.