Fighters from the Islamic State of Iraq and the Levant (Isis) have seized the Syrian city of Palmyra, home to a Unesco world heritage site, putting nearly half of Syrian territory in the jihadi group's hands and sparking fears that treasured antiquities may be destroyed.

Isis announced it had "complete control" of the city on Thursday, and state television said President Bashar al-Assad's forces had withdrawn from the city, which is known to most Syrians by its Arabic name Tadmur.

Ancient Palmyra is known to the world for its iconic avenue of Roman columns, and was a cultural crossroad of the ancient world. The city dates back to the 1st century, when it was an oasis on a trade route linking eastern civilisations with the Roman empire. Its ruins lie to the southwest of modern Tadmur.

"Palmyra is an extraordinary World Heritage site in the desert and any destruction to Palmyra [would be] not just a war crime but . . . an enormous loss to humanity," Unesco head Irina Bokova said in a video.