According to his family, the 32-year-old orthopaedic surgeon from south London had travelled to the city of Aleppo last year to help civilians.
Khan, a father of two who worked at the Royal National Orthopaedic Hospital in London, was arrested by government forces within 48 hours of arriving in Aleppo to work as an emergency surgeon. His mother, earlier this year, had found him in a prison in Damascus weighing just five stone (32kg) and barely able to walk.
The jailed surgeon then claimed he had been tortured while being detained without charge and held for much of the time on his own.
The Syrian National Security Agency had promised his release this week but Monday it said he had died, Afroze Khan, the surgeon's brother, said.
"My brother was going to be released at the end of the week. We were given assurance by the Syrian government," Afroze Khan said. "My brother knew that. He was ready to come back home...He was happy and looking forward to being released."
"We are devastated, distraught and we are angry at the Foreign Office for dragging their feet for 13 months," he added.
The British Foreign Office said: "We are extremely concerned by reports that a British national has died in detention in Syria.
"We are urgently seeking clarification of this from the Syrian authorities. If these tragic reports are true, responsibility for Dr Khan's death lies with them and we will be pressing for answers about what happened, " a spokesperson of the foreign office said.
Afroze Khan said his surgeon brother was moved from a prison in Damascus to the National Security Agency's headquarters. He said his mother, who has been in Damascus for the last four months, was told she could visit her son Monday. But when she arrived she was told he was dead.
--IANS (Posted on 17-12-2013)