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The incident happened on Monday as the vessel attempted to draw closer to the USS Mahan despite the destroyer trying to turn away from it, said Ian McConnaughey, a spokesman for the Bahrain-based 5th Fleet.

The "Mahan made several attempts to contact the Iranian vessel by bridge-to-bridge radio, issuing warning messages and twice sounding the internationally recognised danger signal of five short blasts with the ship's whistle, as well as deploying a flare to determine the Iranian vessel's intentions," McConnaughey said in a statement to AP.

The Iranian vessel came within 1,000 metres of the Mahan during the incident, the lieutenant said. The vessel later turned and sailed away.

Iranian authorities did not immediately report the incident on Wednesday.

The US and Iran routinely have tense encounters in the Persian Gulf and the nearby Strait of Hormuz, through which a third of all oil traded by sea passes.

Iran views the American presence as a provocation and its paramilitary Revolutionary Guard shadows US Navy ships in the Gulf, occasionally firing missiles or rockets nearby.

Since the nuclear deal with world powers, the hard-line Revolutionary Guard has stepped up its encounters with the Americans.

The Navy says it has recorded 35 instances of what it describes as "unsafe and/or unprofessional" interactions with Iranians forces in 2016, compared to 23 in 2015.

With Monday's event, there have been seven so far in 2017, McConnaughey said.

Of the incidents last year, the worst involved Iranian forces capturing 10 US sailors and holding them overnight.