US forensics team in Syria searching for bodies of American hostages murdered by IS 'Beatles'

A forensic team from the US has been deployed to Syria to search for the remains of American hostages murdered by the Islamic State group over the past four years.

The investigators have scoured a mass grave outside the IS group's former self-declared "capital" Raqqa, close to where the Pentagon believes two US hostages were executed.

They face difficulties in their task with the bodies potentially disposed in a wide area and among a huge number of other bodies, CNN reported.

Once it became apparent that IS' so-called "caliphate" was crumbling - following a large-scale US-backed Kurdish offensive on Raqqa - IS took steps to disguise burial sites to make it more difficult for investigators to recover victims' bodies.

Among the Americans beheaded by IS in Syria were journalists James Foley and Steven Sotloff, along with aid worker Peter Kassig. 

Other Westerners were also brutally murdered by the group on video that were distributed among jihadi media channels.

Their killers were the so-called "Beatles" terror cell, a group of four British IS members who carried out the torture and killings of hostages.

Two of the notorious foreign fighters are currently being held by Kurdish forces and are believed to have been questioned on the location of the graves.