Australia's most highly decorated former SAS soldier, Ben Roberts-Smith, is being investigated by the Australian Federal Police for allegedly kicking a handcuffed detainee off a small cliff in an Afghan village in 2012.

Multiple serving and former defence sources in Australia and Afghanistan have confirmed that Mr Roberts-Smith, a Victoria Cross recipient, is the subject of the AFP inquiry into the brutalisation of Afghan farmer and father Ali Jan after he was taken into custody by the SAS on September 11 that year.

On Friday, The Age and The Sydney Morning Herald reported that federal police detectives had travelled earlier this year to Afghanistan and gathered eyewitness testimony from people in and around the village of Darwan that implicates Australian special forces soldiers in the alleged brutal assault and murder.

SAS personnel and support staff have also given eyewitness statements to federal police agents about the treatment of Ali Jan or the operation in which he was killed, according to defence sources who cannot be named because they are not authorised to speak publicly. Under international law, detainees must be treated humanely and be protected from any act of violence.