Friends of American ISIS-held aid worker Peter Kassig call for his release

Colleagues of US aid worker Abdul-Rahman Kassig carry signs during a news conference calling for his release in Tripoli, northern Lebanon on Nov 8, 2014. Kassig, a 26-year-old humanitarian worker from Indiana whose first name was Peter before he conv
Colleagues of US aid worker Abdul-Rahman Kassig carry signs during a news conference calling for his release in Tripoli, northern Lebanon on Nov 8, 2014. Kassig, a 26-year-old humanitarian worker from Indiana whose first name was Peter before he converted to Islam while in captivity, was abducted on Oct 1, 2013. He is currently being held hostage by Islamic State militants, who threatened to behead him in a video issued last month that showed the beheading of British aid worker Alan Henning, 47. The words on the 3rd left read, "Justice to Abdul-Rahman". -- PHOTO: REUTERS

TRIPOLI, Lebanon (AFP) - Friends and former colleagues of a US aid worker whom the Islamic State in Iraq and Syria (ISIS) group has threatened to execute appealed on Saturday for his release.

Twenty-six-year-old Peter Kassig, who converted to Islam and took the Muslim name Abdel Rahman, has been an ISIS captive since 2013.

"We call on IS... to free Abdel Rahman," Firas Agha, a Syrian refugee living in Tripoli who shared a flat with Kassig when he lived and worked in the northern Lebanese coastal city, told a news conference.

"Islam does not allow Muslim to kill Muslim, especially if the Muslim in question has done good work," he said.

In an Oct 3 video showing British aid worker Alan Henning's beheading, the threat was made that Kassig would be next.

The group says its brutal executions are in retaliation for US-led air strikes targeting militants in Syria and Iraq.

Before travelling to rebel-held areas in Syria, Kassig worked in hospitals and clinics treating Syrians forced to flee their war-torn country to neighbouring Lebanon and Turkey.

He made two separate trips into rebel-held areas of Syria before travelling to the eastern province of Deir Ezzor in autumn 2013, when he was taken hostage.

Kassig "was a very enthusiastic young man, so much that he would help refugees out of his own pocket", said Agha.

The former US soldier left the army after fighting in Iraq.

"He told us many times about his dismay over what he saw, both in terms of the killing and destruction," said Agha.

Another Syrian, Dr Ahmad Obeid, told reporters Kassig "cared a lot about giving humanitarian and medical aid to Syrian refugees".

A third refugee, who identified himself only as Mohammed and who now lives in Switzerland, made an emotional appeal.

With the green, black, red and white flag Syrians opposed to President Bashar al-Assad's regime have adopted behind him, Mohammed said he warned Kassig about returning to Syria because he sensed his life would be in danger.

"But Abdel Rahman was convinced of the need to help the Syrians inside Syria, because they need that," he said.

Hostages threatened at the end of four previous ISIS videos have all subsequently been murdered.

Activists say the militants are holding hundreds of hostages, mostly Syrians.