22 December 2013
Last updated at 13:22 ET
David Cameron has written to the mother of a British surgeon who died in a Syrian prison calling his death a “sickening and appalling tragedy”.
The British prime minister criticised the “despicable treatment” Abbas Khan had received and said the Syrian “regime should answer for his death”.
The body of Mr Khan, 32, was flown back to the UK on Sunday ahead of a post mortem on Monday.
His family say the death last week was murder.
Syrian authorities have said Mr Khan committed suicide but the family reject this and the UK Foreign Office has said he was “in effect murdered”.
In the letter dated 20 December and given to the BBC by the family’s lawyer, Mr Cameron told Mrs Khan he and his wife Samantha were “very sorry” to hear of her son’s death.
“I know from my own experience of losing a child that words are of little comfort at this terrible time,” he wrote.
“Abbas’ death is a sickening and appalling tragedy and it is right that the Syrian regime should answer for it.”
He told Mrs Khan the regime’s treatment of her son and “refusal to engage” to allow the UK to support him were “utterly unacceptable”.
In a hand-written ending, he added: “You are in our thoughts and prayers.”
David Cameron wrote to Fatima Khan about her son’s death
Mr Khan’s brother Afroze told the BBC he was “grateful” that Mr Cameron had condemned the Syrian regime over the death.
But he said the prime minister must now “follow through” and use every possible method to get “answers” for the family and bring to justice “those who have murdered my brother”.
“We are very angry at the failures of the British government at securing the release of my brother and now that he is dead we want to see the British government act decisively to respond to the death of a British citizen at the hands of a foreign regime,” he said.
“There must be a firm response to such a callous and cruel act.”
Mr Khan’s brother Afroze said his family was “angry” with the British government
Speaking at Heathrow Airport where Mr Khan’s body arrived, his sister Sara said the British government must co-operate with her family.
“This is the least they can do,” she said.
“They were supposed to be helping us with the repatriation but I think that was left to the undertakers in Beirut.
“We have received letters from the prime minister who said that he’s now going to help us get answers but nothing’s actually been said as to how they expect to do this and when.”
Mr Khan, from Streatham, south London, was arrested in Syria 48 hours after arriving in the country to work in a field hospital in a rebel-controlled area in November last year.
He entered without a visa, but he later told his family he was “accused of treating dying civilians, (women and children), which has been classed as an act of terrorism”.
His mother Fatima, who spent four months in Damascus trying to help him, found him in a prison in the city weighing just five stone (32kg) and barely able to walk.
She told the BBC she was surprised the Syrian regime “cannot differentiate between a humanitarian aid worker and a terrorist”.
The Syrian government said Mr Khan killed himself in his cell but his family believe he was murdered by Syrian intelligence services – just days before he was due to be released.
The family’s solicitor, Nabeel Sheikh, said: “It’s inconceivable that somebody who’s about to be released after a period of 13 months being incarcerated in Syria is going to commit suicide.”
He said Scotland Yard was “actively involved” because Mr Khan’s death was regarded as suspicious.
A Metropolitan Police spokesman said its Counter Terrorism Command was providing family liaison support and would “seek to assist the coroner when appropriate”.