(CNN) An Iranian Islamic Revolutionary Guard Corps ship acted "unprofessional but also provocative" while approaching the USS Mahan, an American destroyer in the Persian Gulf on Monday, according to a US official.

The official said the Iranian vessel had its weapons manned and came within approximately 1,000 yards of the US destroyer.

The Mahan attempted bridge-to-bridge communication with the Iranians but got no response.

The US destroyer then fired a flare but despite the obvious signs from the Mahan, the Iranian ship continued on its course, forcing the US ship to alter direction, the official said.

But he said despite the provocations, the Mahan did not fire any warning shots.

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