A U.S. Navy coastal patrol ship changed course after a fast-attack craft from Iran's Islamic Revolutionary Guard Corps came within 91 meters of it in the central Gulf on Sunday, September 4, U.S. Defense Department officials said on Tuesday, according to Reuters.

It was at least the fourth such incident in less than a month. U.S. officials are concerned that these actions by Iran could lead to mistakes.

Years of mutual animosity eased when Washington lifted sanctions on Tehran in January after a deal to curb Iran's nuclear ambitions. But serious differences still remain over Iran's ballistic missile program, and over conflicts in Syria and Iraq.

Pentagon spokesman Captain Jeff Davis said the Iranian vessel sailed directly in front of the USS Firebolt, forcing the 53-metre U.S. ship to change course.

The incident began when seven Iranian ships "harassed" the Firebolt, Davis said.

A U.S. Defense official, speaking on the condition of anonymity, said the interaction was "unsafe and unprofessional due to lack of communications and the close-range harassing maneuvering," adding that uncovered and manned weapons were seen on the Iranian vessel.

The U.S. ship tried to communicate with the Iranian ship by radio three times but received no response, Reuters says.

The U.S. official said there have been 31 similar interactions with Iranian ships this year, almost double the amount from the same period last year.