A US navy destroyer fired warning shots at four Iranian fast-attack vessels near the Strait of Hormuz.
US defence officials told Reuters that the incident occurred on Sunday (8 January), when USS Mahan established radio communication with the Islamic Revolutionary Guard Corps boats, but they did not respond to requests to slow down as they approached.
The destroyer fired warning flares and a US Navy helicopter deployed a smoke float before the warning shots were fired.
The Iranian vessels came to within 800 metres of the destroyer, which was escorting two other US military vessels.
The incident comes with US president-elect Donald Trump due to take office on 20 January. Trump said on the campaign trail that Iranian vessels threatening US ships in the Gulf would be "shot out of the water".
He has also signalled that he would be prepared to scrap the Iran nuclear deal negotiated by the administration of outgoing President Barack Obama, which saw Iran open its nuclear facilities to international inspectors in exchange for economic sanctions being dropped.
In January 2016, Iran freed 10 US sailors after briefly detaining them when their vessel allegedly strayed into Iranian waters.