A U.S. Navy coastal patrol ship was forced to changed course after being harassed by fast-attack craft from Iran's Islamic Revolutionary Guard Corps, officials say.

The Iranian vessel came within 100 yards of the U.S. boat in the central Gulf on Sunday, U.S. Defense Department officials said.

It is at least the fourth such incident in less than a month as defense officials have warned such confrontations have doubled since last year.

The USS Firebolt was forced to change course after an Iranian Navy patrol boat came within 100 yards of it on Sunday despite repeated warnings

It comes after the USS Nitze fired flares at Iranian patrol boats after the approached at speed and in an 'unsafe and unprofessional' manner, officials said

That is despite the Iranian nuclear deal and billions of dollars handed over to Iran in withheld sanction cash.

Authorities are concerned that reckless actions by Iranian vessels could lead to mistakes and potentially spark a conflict.

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 174ft U.S. ship to change course.

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

Authorities say confrontations between Iranian and American naval vessels in the Persian Gulf have doubled since last year despite the nuclear deal

Officials say the Nitze made repeated attempted to contact the approaching Iranian vessels but received no response

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.

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.

'We don't see this type of unsafe and unprofessional activity from any other nation,' the defense official added.

In late August, a U.S. Navy patrol craft fired warning shots toward an Iranian fast-attack vessel that approached two U.S. ships.

The USS Squall was also forced to take evasive action and fired warning shots at Iranian vessels that approached at high speed, officials said

The confrontations are taking place in a narrow band of international water that runs between Iran on one side and the United Arab Emirates on the other

At the time, Iran's defense minister said Iranian vessels were just doing their job.

Last week, the head of U.S. Central Command, General Joseph Votel, said unsafe maneuvers in the Gulf were part of the Iranian regime trying to exert its influence in the region.

Kenneth Pollack, a former top CIA and White House official, said that one or two incidents could have been explained 'as being the work of an over-zealous commander' within Iran's Islamic Revolutionary Guard Corps.

But the number of such incidents in recent weeks make it 'very hard for me to believe these are not sanctioned by higher authority' within Iran, said Pollack, a senior fellow at the Brookings Institution.