The United States says boats from Iran's Islamic Revolutionary Guards Corps (IRGC) harassed two U.S. Coast Guard ships in the Persian Gulf early this month, in what the U.S. Navy described as the first such incident in a year.
The U.S. Navy said on April 27 that the tense, April 2 encounter resulted in no injuries or damage.
Iran did not immediately acknowledge the incident, which occurred as Washington and Tehran announced they would conduct indirect negotiations aimed at reviving the 2015 Iran nuclear deal.
The talks between Iran and world powers began in Vienna earlier this month and are to resume in the Austrian capital on April 27.
U.S. Navy officials said that three Iranian fast-attack craft and a support vessel known as Harth 55 swarmed two Coast Guard ships while they were patrolling international waters in the Persian Gulf.
The Harth 55 ship repeatedly crossed in front of the bows of USCGC Monomoy and USCGC Wrangell, forcing the Coast Guard vessels to come to make defensive maneuvers to avoid collision, the officials said.
"The U.S. 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 maneuvers," according to Commander Rebecca Rebarich, a spokeswoman for the U.S. Navy's Bahrain-based 5th Fleet.
"After approximately three hour of the U.S. issuing warning and conducting defensive maneuvers, the [Iranian] vessels maneuvered away from the U.S. ships and opened distance between them."
The interaction marked the first "unsafe and unprofessional" incident involving Iranian warships since April 15 last year, Rebarich added.
It followed a series of incidents across the Middle East attributed to a shadow war between Iran and its foe Israel, which included attacks on cargo ships and alleged sabotage at Iran's Natanz nuclear site.