The controversial New York lawyer Stanley Cohen led unsuccessful negotiations to intervene with Islamic State in an attempt to save the life of the American hostage Peter Kassig, a British newspaper reports today.

According to The Guardian, the talks with the spiritual leaders of Islamic State began in mid-October and lasted for several weeks until Kassig was murdered by beheading in November at the age of just 26. The FBI was aware of the talks.

Cohen has represented Osama bin Laden's son-in-law and members of Hamas in court. He had persuaded senior al Qaeda clerics to attempt to intervene with Islamic State to save Kassig's life.

Kassig had done relief and medical work in the region and helped raise money for the refugees before he was kidnapped in October 2013.

The Guardian reports that although the US would never pay a ransom or agree to a prisoner swap, or agree to stop bombing ISIS, Cohen suggested to Islamic State that Kassig be released as a political gesture to other Muslim political prisoners around the world, including in Guantánamo. Kassig himself had, while captive, converted to Islam and had taken the name Abdul-Rahman.

As part of the deal, Cohen attempted to prevent the detention of Abu Muhammed al-Maqdis, the Salafist preacher, in Jordan. Talks to save Kassig collapsed when al-Maqdis was detained.

Mr Cohen is due to serve a prison sentence after pleading guilty to federal tax offences in April.