A US general on Tuesday said Tehran is acting in an unsafe and provocative manner in the Persian Gulf, where the Iranian Revolutionary Guard Corps (IRGC) frequently conducts risky maneuvers around US vessels.

The comments from General Joe Votel, head of the US military's Central Command, came after the Pentagon last week denounced a series of "unprofessional" encounters in the Gulf, including one that prompted an American ship to fire warning shots.

"In recent days, we have witnessed even more provocative activity by the IRGC and navy vessels," Votel said.

"That type of behavior is very concerning, and we hope to see Iran's naval forces act in a more professional manner."

Votel noted that 90 percent of unsafe encounters with Iranian boats are conducted by Quds specials forces from the IRGC.

"I am concerned about rogue commanders, rogue Iranian Quds force naval commanders who are operating in a provocative manner and are trying to test us," he said.

The most dramatic moment in four encounters the Pentagon described last week occurred when an IRGC boat maneuvered around two US patrol ships, the USS Squall and USS Tempest.

Despite multiple warnings via radio and loudspeaker, the Iranian boat approached head on, coming within 200 yards (meters) of one of the vessels.

Ultimately, the Squall fired three warning shots from a 50-caliber gun, causing the Iranian vessel to turn away, officials say.

A defense official told AFP that ships from the US and Iranian navies had interacted more than 300 times in 2015 and more than 250 times the first half of this year.

Ten percent of those encounters were deemed unsafe and unprofessional, the official said.

"What we see with the Iranians is not particularly responsible. It is provocative, in some cases; it's unsafe," Votel said.

"And it can lead to situations where we may not be able to de-escalate in a time before something happens."