The U.S. and Iran on Saturday reported their second confrontation this week in the waters off the Persian Gulf, as political tensions between the longtime rivals flare.

The U.S. Navy said several vessels operated by Iran’s powerful Islamic Revolutionary Guard Corps in the Gulf approached American ships at high speed on Friday.

After the U.S. Navy failed to reach the Iranian ships by radio, a Navy helicopter fired warning flares at a safe distance, according to the U.S. Naval Forces Central Command.

The Iranian ships subsequently stopped moving toward U.S. ships, it said, describing the interaction as “safe and professional.”

The IRGC, however, said an American Nimitz-class aircraft carrier accompanied by a warship came near IRGC boats patrolling Iran’s Resalat oil-and-gas field. The aircraft carrier flew a helicopter close to the Iranian vessels on Friday afternoon, according to the official Islamic Republic News Agency.