The three fast inshore attack craft operated by the Iranian Islamic Revolutionary Guard Corps Navy came within 68 yards (62 meters) of the USS Firebolt and the US Coast Guard vessel Baranoff on Monday in international waters in the northern part of the Gulf, the Navy said.

"The US crews issued multiple warnings via bridge-to-bridge radio and loud-hailer devices," a statement from the US Fifth Fleet said.

As the Iranian boats continued to move closer, "The crew of Firebolt then fired warning shots, and the IRGCN vessels moved away to a safe distance from the US vessels," it said.

The US side said the Iranian actions "increased the risk of miscalculation and/or collision," and warned that the US ship commanders "retain the inherent right to act in self-defense."

On April 2 four Revolutionary Guard vessels, three fast attack craft and on large Harth 55 catamaran, similarly came close to two US Coast Guard patrol ships, crossing their bows while ignoring warnings, the Fifth Fleet said earlier Tuesday.

US navy footage showed the Harth 55 cutting in front of a US vessel, which abruptly steered off course to avoid collision.

The confrontations came amid talks in Vienna between Iran and major powers on the mechanics of a US return to a landmark 2015 nuclear deal abandoned three years later by then president Donald Trump.

In offering to rejoin the agreement in February, US President Joe Biden stressed the need to reduce "Iran's destabilizing activities across the Middle East."