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.