Iranian military vessels had a close encounter with a U.S. surveillance ship on Saturday in what U.S. officials tell CBS News was both an “unsafe” and “unprofessional” maneuver by the Iranians in the strategic Strait of Hormuz.

CBS News’ David Martin reports that the USNS Invincible, which is outfitted with sonar to track submarines and radar to monitor missile tests, was transiting the Strait of Hormuz when it was forced to change its course to avoid a small group of Iranian Revolutionary Guard fast boats that had positioned themselves in front of her.

The “unsafe” encounter came just two days after an Iranian navy frigate came within 150 yards of the Invincible in the Gulf of Oman, just south of the Strait of Hormuz. The gulf separates Oman from southeastern Iran.

The earlier encounter was deemed “unprofessional” but not unsafe because the frigate maintained a parallel course with the Invincible at the point of closest approach. The Saturday incident was also more worrying as it involved the elite Revolutionary Guard unit.

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Still, the Saturday encounter was not the most serious of 2017. In January, a U.S. Navy destroyer fired multiple warning shots at Iranian patrol boats as they sped toward the destroyer in the Strait of Hormuz “with their weapons manned.”

The crew of the USS Mahan fired the warning shots after attempting to establish contact with the Iranians and after dropping smoke flares, U.S. officials said.

The U.S. Navy occasionally has confrontations with Iranian naval forces in the Persian Gulf but they do not usually reach the point of prompting warning shots by the U.S.

A U.S. Navy official told CBS News the Mahan was transiting the Strait of Hormuz -- through which nearly a third of all oil traded by sea passes -- when the Iranian boats sped toward it and failed to halt despite U.S. cautionary moves.

In May of 2016, the commander of the Revolutionary Guard threatened to close the Strait of Hormuz to the United States and its allies if they “threaten” the Islamic Republic.