The US Navy has accused Iran's Islamic Revolution Guard Corps (IRGC) of "dangerous and provocative actions" in international waters in the Gulf.
Eleven IRGC Navy vessels repeatedly conducted "harassing" approaches of six US Navy and Coast Guard ships on Wednesday, it said. One passed within 10 yards (9m) of a Coast Guard cutter.
The US vessels were involved in a military exercise with US Army Apache attack helicopters.
There has been no word yet from Iran.
The incident came a day after armed men - believed to have been IRGC personnel - seized a Hong Kong-flagged tanker in the Gulf of Oman and redirected it into Iranian waters before releasing it.
The US Navy said that during Wednesday's exercise the IRGC vessels crossed the bows and sterns of the US vessels at "extremely close range and high speeds, including multiple crossings of the [USS Lewis B] Puller with a 50 yard closest point of approach and within 10 yards of [USCGC] Maui's bow.
"The US crews issued multiple warnings via bridge-to-bridge radio, five short blasts from the ships' horns and long-range acoustic noise maker devices, but received no response from the IRGCN."
"After approximately one hour, the IRGCN vessels responded to the bridge-to-bridge radio queries, then manoeuvred away from the US ships and opened distance between them," it added.
BREAKING: 11 Iranian #IRGCN vessels repeatedly conducted dangerous & harassing approaches against U.S. naval ships operating in international waters of North Arabian Gulf. U.S. crews took actions deemed appropriate to avoid collision.— U.S. Navy (@USNavy) April 15, 2020
Details: https://t.co/ZVKPKv738o pic.twitter.com/lKJgDz0l2N
The US Navy said such "dangerous and provocative actions increased the risk of miscalculation and collision", and were not in accordance with international maritime conventions or customs.
Last year, Iranian forces seized a British-flagged tanker in retaliation for the seizure of an Iranian tanker by the British territory of Gibraltar. Both vessels were eventually released.
The US also accused Iran of carrying out attacks on six tankers in the Gulf of Oman and launching missiles and drones at two oil facilities in Saudi Arabia. Iran denied any involvement.
Tensions between the US and Iran rose further in January, when the US killed a top IRGC general in a drone strike in Iraq. Iran responded by launching missiles at Iraqi military bases hosting US forces.