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By Idrees Ali

WASHINGTON, Aug 25 (Reuters) - A U.S. Navy ship fired warning shots after an Iranian fast-attack craft approached two U.S. ships in the northern Gulf, a U.S. defense official said on Thursday, in the most serious of a number of incidents in the same area this week.

The official, speaking on the condition of anonymity, said the USS Squall patrol craft fired three warning shots from a .50 caliber gun after warning flares did not work.

The incident, which happened on Wednesday, started with three Iranian vessels, but there was only one around by the time the warning shots were fired, the official said. He described the Iranian behavior as "unsafe, unprofessional, and not routine."

At one point, the Iranian vessel came within 200 yards (193 meters) of a U.S. ship, the official said.

Another interaction took place between an Iranian and U.S. ship on Wednesday, the defense official said but gave few more details.

The incidents come after the Pentagon accused Iranian vessels of harassing a U.S. warship near the Strait of Hormuz on Tuesday.

Earlier on Thursday, Iran's defense minister said those Iranian vessels were just doing their job.

"If an American ship enters Iran's maritime region, it will definitely get a warning. We will monitor them and, if they violate our waters, we will confront them," Defense Minister Hossein Dehghan said in a statement reported by the Iranian Students' News Agency.

A State Department spokeswoman said it was not clear what the intentions of the Iranian ships were, but the behavior was unacceptable.

"We believe that these type of actions are of concern, they unnecessarily escalate tensions," State Department spokeswoman Elizabeth Trudeau told a briefing.

(Reporting by Idrees Ali; Additional reporting by Mohammad Zargham; Editing by Paul Simao and Alistair Bell) ((mohammad.zargham@thomsonreuters.com;))