The Iranian Revolutionary Guard Corps sailed several fast-attack vessels near a U.S. Navy ship in the Strait of Hormuz during the weekend, forcing the U.S. ship and its entourage to change course, a U.S. official said.

Reuters reported the Revolutionary guard vessels sailed within 600 yards of the USNS Invincible tracking ship. The Invincible was flanked by three British Royal Navy ships.

A U.S. official described the Iranian action as “unsafe and unprofessional.” Efforts to contact the Iranian vessel by radio were unsuccessful, the official said.

The incident occurred Saturday, just two days after an Iranian frigate, sailing on a parallel course, came within 150 yards of the Invincible in the Gulf of Oman. U.S. officials labeled that encounter “unprofessional.”

The Invincible is outfitted with submarine-tracking sonar and sophisticated X- and S-band Gray Star radar to monitor missile tests, according to War is Boring.

The two encounters are the latest in a series of incidents. In January, a U.S. Navy destroyer USS Mahan fired several warning shots at Iranian patrol boats that came toward it in the Strait of Hormuz. In November a small Iranian vessel allegedly pointed a gun at a U.S. Navy helicopter in the strait. There also have been several encounters in the Persian Gulf.

The Iranian Revolutionary Guard has said it would close the Strait of Hormuz, most recently last May, if the face of threats from the United States and its allies. Nearly a third of all oil traded by sea moves through the waterway.

Tensions with Tehran have eased little since agreement over its nuclear program was reached with the five permanent members of the U.N. Security Council plus Germany and implemented little more than a year ago. Iranian officials have complained the West has failed to live up to its end of the deal while the West has expressed unhappiness with Iranian missile tests.

The Invincible is a 224-foot-long Stalwart class ocean surveillance ship, displacing 2,800 tons. It belongs to the Military Sealift Command, a quasi-civilian agency operated by the Navy, and is maintained by 18 civilian contractors. The vessel typically is deployed in the Persian Gulf and Indian Ocean.