Isis: President Obama Confirms 'Evil' Murder of US Aid Worker Peter Kassig
US humanitarian Peter Kassig was executed by militants from the Islamic State. Kassig family/Facebook

US officials backed talks with top jihadi clerics and former Guantanamo detainees to try to save the life of a US detainee killed by terror group Islamic State (Isis).

Talks to convince IS to spare Peter Kassig's life began in mid-October and lasted for several weeks, the Guardian reported.

The attempt to save the man's life was made upon suggestion of a controversial New York lawyer, Stanley Cohen, who represented Osama bin Laden's son-in-law and members of Hamas in US courts.

Cohen contacted two senior clerics aligned with al-Qaida, Abu Muhammed al-Maqdisi and AbuQatada to intervene with Isis.

The aim of the talks was not only to save Kassig but also to convince IS to stop abducting and executing civilians. If the terror group agreed to do so, Maqdisi, Abu Qatada and other senior religious figures would stop publicly denouncing IS as extremists.

However, during the negotiations, Jordanian security services arrested Maqdisi for "using the internet to promote and incite views of jihadi terrorist organisations" and the agreement collapsed.

Kassig was abducted in October 2013 by IS in Syria, where he had gone to carry out some humanitarian work. During his captivity, he converted to Islam and assumed the name of Abdul-Rahman Kassig.

He was beheaded by a militant known as "Jihadi John" on 16 November 2014. His execution was recorded and the video, Although The Unbelievers Dislike, was uploaded on social media the same day.