The US Navy has accused Iran of repeatedly harassing six of its ships on a training exercise in international waters.

US warships conducting drills with US Army Apache attack helicopters in the North Arabian Gulf were approached by 11 vessels belonging to the Iranian Islamic Revolutionary Guard Corps Navy (IRGCN).

The Iranian ships repeatedly conducted dangerous and harassing approaches to the US vessels, crossing their bows and sterns at close range and high speed, the US 5th Fleet reports.

The American ships involved were the USS Lewis B Puller, USS Paul Hamilton, USS Firebolt, USS Sirocco, USCGC Wrangell and USCGC Maui, all based out of Bahrain.

At one point the Iranian vessels came with 50 yards of the Puller, the Navy’s first purpose-built expeditionary mobile base vessel, and 10 yards of the Maui, a US Coast Guard Island-class Cutter.

US crews issued multiple warnings via bridge-to-bridge radio, five blasts from the ships’ horns and long-range acoustic noise maker devices.

They received no response from the Iranians for approximately one hour.

The IRGCN vessels then responded to the bridge-to-bridge radio queries and manoeuvred away from the US ships, opening up distance between them.

A US Navy statement says that the Iranians' “dangerous and provocative actions increased the risk of miscalculation and collision” and were not in accordance with the “rules of the road” or internationally recognised maritime customs.

The Navy, Coast Guard, Marines and Army have been conducting joint interoperability operations near Iran in the North Arabian Gulf since late March.