A serving British soldier has joined Kurdish Peshmerga forces fighting Islamic State insurgents, Sky television reported on Thursday.
Sky said the man, who is not being named, had told family and friends he intended to spend a year in the Middle East.
He had been on leave from the British Army at the time he left for the region, it added.
A British Ministry of Defense spokesman said: "We are aware of these reports and we are looking into them."
Sky said the man had been in the army since leaving school at 16 and was known to have been been learning Arabic. It was not known exactly where he had gone.
No further details were immediately available.
The man would be the first serving soldier to have joined Kurdish forces, but a small number of former British servicemen have already done so.
Security forces also estimate some 600 British Muslims have travelled to Syria to join the conflict there, some of them with the militant Sunni Islamist group Islamic State.
Around half have since returned, and dozens have been arrested in Britain under anti-terrorism legislation.
Peshmerga forces have been fighting ISIS in the Kurdish region of northern Iraq and trying to surround the city of Mosul to cut off ISIS supply lines from Syria.
Islamic State has caused international alarm by capturing large expanses of Iraq and Syria, declaring an Islamic "caliphate" and erasing borders between the two.
British forces have been taking part in air raids on ISIS in Iraq but only a handful of troops remain on the ground in the country, in a training capacity.
Nearly all other British forces were pulled out of Iraq in 2011, eight years after the U.S.-led invasion that brought down Saddam Hussein.
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