LONDON, Sept 18 (Reuters) - Muslim groups across Britain called on Thursday for the release of British hostage Alan Henning, a volunteer aid worker captured in Syria whose life was threatened in a recent video released by Islamic State militants.
Henning was part of an aid convoy taking medical supplies to a hospital in northwest Syria in December last year when it was stopped by gunmen and he was taken away.
In a video released on Saturday which showed the beheading of fellow British aid worker David Haines, an Islamic State militant threatened to kill Henning if Prime Minister David Cameron continued to support the fight against them.
The man in the video is himself believed to be a British national and has been dubbed "Jihadi John" by UK media.
"We, the undersigned British Muslim Imams, organisations and individuals, wish to express our horror and revulsion at the senseless murder of David Haines and the threat to the life of our fellow British citizen, Alan Henning," said a letter signed by more than 100 Muslim groups and leaders.
"The un-Islamic fanatics are not acting as Muslims, but as the Prime Minister has said, they are acting as monsters. They are perpetrating the worst crimes against humanity. This is not Jihad - it is a war against all humanity," added the letter sent to the Independent newspaper.
Moved by the plight of Syrians, taxi driver Henning, 47, known as "Gadget" because of his technical skills, had volunteered to join humanitarian missions from northwest England where he lived and even had "aid for Syria" tattooed on his arm.
"It's all worthwhile when you see what is needed actually get to where it needs to go," Henning said in a video broadcast by the BBC. "No sacrifice we do is nothing compared with what they are going through every day."
Majid Freeman, who was there when Henning was captured, said the militants had concluded his friend was a spy. "Please, please, please show him some mercy and understand he's a humanitarian aid worker, he's not a fighter," he told BBC TV.
Britain said on Monday no immediate rescue mission by special forces was possible because it was not known where Henning was being held. Cameron has so far resisted joining the United States in carrying out air strikes against IS, but could do so once an international coalition has been formed.
The call to release Henning came from across Muslim communities in Britain, including some organisations that are highly critical of British foreign policy and blame western interference for fanning the recent crisis in Iraq and Syria.
"Alan Henning went to Syria with Muslims and is known to have been helping the people of Syria," said Asim Qureshi, research director at CAGE, which supports communities it says have been affected by anti-terrorism policies.
"He is not involved in any hostility to Islam or Muslims. Therefore, he cannot be considered a prisoner of war under Islamic law and should be released immediately."
Editing by Catherine Evans