Army sappers were defusing mines and bombs planted by IS in the city's ancient ruins, a UNESCO world heritage site where the militants sparked a global outcry with the systematic destruction of treasured monuments.

"After heavy fighting during the night, the army is in full control of Palmyra -- both the ancient site and the residential neighbourhoods," a military source told reporters.

IS fighters pulled out, retreating towards the towns of Sukhnah and Deir Ezzor to the east. IS overran the Palmyra ruins and adjacent modern city in May 2015.

It has since blown up two of the site's treasured temples, its triumphal arch and a dozen tower tombs, in a campaign of destruction that UNESCO described as a war crime punishable by the International Criminal Court.

The militants used Palmyra's ancient amphitheatre as a venue for public executions, including the beheading of the city's 82-year-old former antiquities chief.

The oasis city's recapture is a strategic as well as symbolic victory for President Bashar al-Assad, since it provides control of the surrounding desert extending all the way to the Iraqi border.

IS, behind a string of attacks in the West including last week's Brussels bombings, is under growing pressure from Syrian and Iraqi military offensives to retake key bastions in its self-proclaimed "caliphate".

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