Syrian regime forces backed by heavy Russian airstrikes have recaptured the ancient city of Palmyra from Islamic State fighters, in what monitors said was the single biggest defeat for the jihadists since 2014.

Victory was declared on Syrian television early on Sunday morning, the army describing it as a "mortal blow" to the jihadists who seized the city last year and dynamited its ancient temples.

"Palmyra will be the central base to broaden operations... against Daesh in numerous areas, primarily Deir Ezzor and Raqqa," the army said, vowing to "end" the terrorist group in its stronghold.

The recapture of Palmyra, a strategic oasis city and UNESCO World Heritage site known as "the pearl of the desert", follows a three week offensivebacked by Russian fighter jets and helicopters, despite Moscow having announced the withdrawal of most of its forces a fortnight ago.

Battles raged for three days inside the city after regime forces entered it on Thursday. With smoke billowing from a central district, television footage from Palmyra’s citadel on Saturday showed a soldier waving a Syrian national flag near the medieval castle walls.

The London-based Syrian Observatory for Human Rights reported on Sunday morning that there was still gunfire in the eastern part of the city as Isil forces retreated.