SERA Abu Omar Aqidi, a prominent member of Jabhat al-Nusra, the al-Qaeda linked Syria group, recalls being treated by Peter Kassig for a shrapnel wound

A senior al-Qaeda jihadist has spoken out in defence of a Western hostage slated as the next to be beheaded by the rival extremists Isil in Syria.

Abdul-Rahman Kassig, the American Muslim convert and aid worker formerly known as Peter, has been named by Islamic State of Iraq and the Levant as the next victim in their series of videoed murders of British and American captives.

Isil members warned on Twitter that Mr Kassig was to be killed on Wednesday, as a "deadline" passed for meeting their demands – that America call off its bombing campaign.

But the Isil threat to Mr Kassig has caused dissent from within the ranks of the jihadists' own community, with a number of Islamists speaking out in support of him, particularly because of his conversion.

Abu Omar Aqidi, a prominent member of al-Qaeda faction Jabhat al-Nusra, said in a public statement that he had been told Mr Kassig was the medic who treated him and other fellow jihadists.

Mr Kassig "performed a successful operation under bombardment by the regime," Aqidi said in a series of tweets, explaining that he had removed a piece of shrapnel from his wound.

He added that Mr Kassig had also treated several of his colleagues, including "Abu Dujana" who he and others have named as leader of the group in Deir al-Zour province in north-eastern Syria.

Jabhat al-Nusra had been a rival of Isil but the American bombardment against both groups has started to bring them closer together again.

Aqidi's tweets suggested that his men had asked after Mr Kassig when he went missing and that it had come as a surprise when "he showed up in the video where ISIS threatened the United States".

Mr Kassig was seized by Isil jihadists on October 1 2013 while delivering medical supplies to Deir al-Zour for the charity that he had established.

Aged just 26, he had moved to the Turkish border town of Gaziantep to found SERA (Special Emergency Response and Assistance), a non-governmental organisation providing first-response humanitarian aid.

Friends describe him as a slightly intense and sincere young man, haunted by his brief experience as a US army ranger in Iraq in 2007, who wanted to "give back". He threw himself wholeheartedly into the organisation, often spending his own money to keep SERA going when funding was low.

He made repeat trips to provide medical support in Syria, long after the frequency of kidnappings of foreigners had stopped other aid workers. Once inside, as Aqidi claimed, he was indiscriminate in treating the war wounded, whether fighters or civilians.

Aqidi said that when we was being treated by Mr Kassig he had believed the doctor was a fellow jihadist, adding that "it later became clear to me that he's an humanitarian activist that served in Deir Al-Zour for more than a year and was then kidnapped by Isil".

Mr Kassig converted to Islam early in his captivity and, according to the accounts of fellow hostages who have since been released, has dedicated himself to practising the religion.

Nonetheless, last month the hostage appeared in a video posted on YouTube, wearing an orange robe and kneeling in the desert whilst a masked extremist promised he would be killed in response to the American air strikes.

Isil has already beheaded four of Mr Kassig's fellow hostages in this fashion, most recently the British aid worker Alan Henning.

On Wednesday, a Norwegian journalist who converted to Islam during a period of captivity by the Taliban in Afghanistan, and who has continued to practise the faith since his release, appealed directly to Isil leader Abu Bakr al-Baghdadi to save Mr Kassig.

Salah ad-Din Refsdal, 51, who was born Paul Refsdal said: "People have suggested that Caliph Ibrahim is not informed that some of his followers are planning to murder Abdul Rahman Kassig.

"That must clearly be the case. Having studied Islam extensively he for sure is aware of how enormous of a sin it would be to murder a brother who just converted to Islam."

Mr Refsdal cited an Islamic ruling condemning the killing of Muslim converts, adding that Baghdadi: "must therefore do the only right thing and stop his followers from murdering our brother Abdul Rahman Kassig".