A British surgeon who was imprisoned in Syria for over a year has died in jail, his brother said.

Dr Abbas Khan, 32, from Streatham, south London, was detained after travelling to Aleppo to work in a field hospital in November last year.

An account run by his brother Afroze, tweeted on Tuesday: "Dear All, I sorrowfully inform you of the news that Dr Abbas Khan was killed yesterday. An innocents [sic] life was taken meaninglessly. He was the best brother I could have ever asked for and I know no one with a purer heart than him.

"His release was due to be this week. I thank you all for your love and support. We still need your help. Abbas is not home. Help us bring him home for the burial he deserves."

He told the BBC his brother, who worked at the Royal National Orthopaedic hospital in London, knew he was to be freed and was happy and looking forward to being released. He expressed anger at the Foreign Office for "dragging their feet for 13 months".

Father-of-two Khan was arrested by government forces within 48 hours of arriving in Aleppo, Syria's biggest city, to work as an emergency surgeon. His family lost contact with him for months but his mother, Fatima, travelled to Damascus earlier this year to try to find out what had happened to him.

When she tracked him down, she found his weight had dropped below 5 stone (32kg) and he was barely able to walk. He told her he had been held for eight months in darkness in an underground cell and tortured. She was told she could visit her son on Monday but when she arrived she was told he was dead, Afroze Khan told the BBC.

The Bradford West MP, George Galloway, said he had been negotiating for months with the Syrian government over the release of Khan and was due to fly out on Friday to bring him home, after receiving a call from the Syrian foreign minister informing him that President Bashar al-Assad had ordered Khan's release.

"This is heartbreaking and devastating news for his family who have been working so hard for so long to secure his release," he said. "Particularly because his freedom had been agreed and he was due to return with me in the next few days. My sincere condolences go out to his family whose pain is unbearable."

Galloway added that the family were kept fully informed of Khan's proposed release before Christmas. Last week, Afroze Khan had warned that his brother was depressed and might want to harm himself.

A spokeswoman for the RNOH said: "The Royal National Orthopaedic hospital is very saddened to hear of the death of Dr Abbas Khan. On behalf of the RNOH, we would like to extend our deepest sympathies to Dr Khan's family at this very difficult time."

The Foreign Office said it was "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".

More than 1,000 detainees are believed to have died in custody in Syria since the uprising against Assad began in March 2011, according to Amnesty International, which says most were from the effects of torture or other ill-treatment.