This comes amid talks in Vienna between Iran and major powers on the mechanics of a US return to a landmark 2015 nuclear deal abandoned by former president Donald Trump.

The US Navy's Fifth Fleet said an Iranian Harth 55 vessel -- a catamaran-type ship -- along with three fast attack crafts, approached the two US coast guard boats conducting routine security patrols on April 2.

"The Harth 55 repeatedly crossed the bows of the US vessels at an unnecessarily close range, including crossings both (US ships) Wrangell and Monomoy's bows at a 70 yard (64 metre) closest point of approach," a US statement said.

"The Harth 55 closed aggressively on Wrangell's bow, resulting in Wrangell manoeuvering to avoid collision while sounding five short blasts from the ship's horn."

US navy footage showed an Iranian ship cut in front of a US vessel, which abruptly moved to avoid collision.

"The US crews issued multiple warnings via bridge-to-bridge radio, five short blasts from the ships' horns, and while the Harth 55 responded to the bridge-to-bridge radio queries, they continued the unsafe manoeuvres," the statement added.

It said the Iranian ships moved away after approximately three hours in the encounter, that the US navy "deemed unsafe and unprofessional".

The remaining partners to the 2015 nuclear deal have been engaged in discussions aimed to return Washington to the accord it withdrew from and to lift the sanctions it reimposed on Iran, and Tehran's return to nuclear commitments it cut in retaliation.

Iran has repeatedly demanded that all US sanctions reimposed since 2018 be lifted, and stressed its readiness to return to nuclear commitments once it has been verified.

A US delegation is present at Vienna but holding no direct talks with Iranians, while the European Union acts as a intermediary and coordinates between the two.