A mass grave containing at least 65 bodies killed by the Islamic State extremist militia in the ancient city of Palmyra has been discovered by Syrian government forces, a monitoring group has reported.

The Syrian Observatory for Human Rights says the bodies belonged to members of regime forces and allied paramilitaries who were executed after Islamic State took over Palmyra in May 2015.

The UK-based watchdog says the grave was found near a military airport in the central Syrian city, but there was no official confirmation in Damascus.

If confirmed, the grisly find will be the second of its kind in Palmyra since it was retaken from Islamic State nearly two months ago.