Syrian state media and an opposition monitoring group say government forces backed by Russian airstrikes have driven Islamic State fighters from the historic central town of Palmyra, held by the extremists since May.

Government forces have been on the offensive for nearly three weeks to try to retake the town, which is home to famed Roman-era ruins and was once one of Syria's top tourist destinations.

State TV quoted an unnamed military official as saying that "the armed forces and groups of popular defense committees have fully taken control of Palmyra." The popular defense committees are militias allied with the government.

The Britain-based Syrian Observatory for Human Rights confirmed IS has lost the town, saying there were many deaths among the extremists.

Syrian president Bashar Assad has described his army's recapture as a "significant achievement."  He said that the overthrow of the Islamic State group in the historic town offered "new evidence of the effectiveness of the strategy espoused by the Syrian army and its allies in the war against terrorism."