TRIPOLI, Lebanon -- Several Syrian friends of an American aid worker held by the Islamic State group and threatened with decapitation called Saturday for his release, saying he converted to Islam and was helping Syrians.

The friends of Peter Kassig, who became Muslim while in captivity and changed his name to Abdul-Rahman Kassig, held a news conference in the northern city of Tripoli, where Muslim militants and Lebanese troops clashed last month.

Kassig, a 26-year-old former Army Ranger, was helping victims of the Syrian civil war when he was captured last year.

One of the friends, Amjad al-Moghrabi, told reporters: "We are demanding the Islamic State to release him, if they know Islam. He is a Muslim and has not participated in what his country is doing." He was referring to airstrikes by the U.S.-led coalition against Islamic State group in Iraq and Syria.

Dr. Ahmad Obeid, a friend of Kassig said "our demand is to release him and to return to his family because as a person he helped us and we should ask for mercy for him."

"He is unfortunately detained so we are calling for his freedom because he supported our cause and we cannot leave him and let them hurt him," Obeid said.

A video released last month appeared to show Kassig, of Indianapolis, kneeling as a masked militant says he will be killed next, after the purported beheading of a British hostage.

Kassig's parents say their son served in the Iraq war and was honourably discharged. Kassig travelled to Lebanon in 2012 to work as a medical assistant in border hospitals and has been held since October 2013 while in Syria.

In the room where the news conference was held, some of Kassig's friends held banners that read in Arabic and English: "Justice for Abdul Rahman."

The Islamic State group has reportedly so far killed four Westerners they were holding.

Britons David Haines, a former air force engineer, and Alan Henning, a taxi driver from northwest England, were beheaded, as was U.S. reporter James Foley and American-Israeli journalist Steven Sotloff.

In addition to Kassig, the Islamic State also holds British photojournalist John Cantlie.