A man who left London to join the Islamic State group in Syria has died while being held in prison in the country, the BBC has been told.
One source said that Ishak Mostefaoui, previously from east London, was killed while attempting to escape custody.
Another said the death came during serious disorder in a jail in Hassakeh, which houses IS prisoners from various countries.
The death and surrounding circumstances have not been officially confirmed.
After being captured last year, the 27-year-old was held in a prison in north-east Syria controlled by the Kurdish-led, US-backed Syrian Democratic Forces.
Sources said he was one of around 10 British men and 30 British women being held by the militia - but he was the first to die in SDF custody.
The prison where he was held is a converted school. When the BBC spoke to him there last October he admitted joining IS.
The general situation in the prisons and camps where IS prisoners are being held has deteriorated this year and there have been several riots.
The British government has refused to allow adult prisoners to return to the UK, saying they should be put on trial in the region.
A government spokesman told the BBC the Foreign Office had advised against all travel to Syria since 2011.
They added: "Those who chose to leave the UK and fight for, or support, Daesh potentially pose a very serious national security risk."