The "Islamic State of Iraq and the Levant" (ISIL) recaptured Palmyra on Sunday (December 11th) after Syrian armed forces pulled out of the desert city, AFP reported.

"Despite the ongoing air raids, ISIL retook all of Palmyra after the Syrian army withdrew south of the city," said Syrian Observatory for Human Rights head Rami Abdel Rahman.

The extremists made a rapid advance across the city after overrunning a northern neighbourhood and capturing the famed citadel to Palmyra's west.

ISIL launched an offensive last week near Palmyra, a renowned UNESCO World Heritage site.

It has killed around 100 members of Syrian government forces since launching simultaneous attacks on several regime positions near Palmyra on Thursday, the Observatory said.

The group targeted areas including near the Mahr and Shaar oil and gas fields and seized government checkpoints, silos and the village of Jazal, north-west of Palmyra, before making a major push into the desert city on Saturday.

But despite continuing air raids and the arrival of Syrian army reinforcements, ISIL seized control of the city hours later, the Observatory said.

Experts said several factors explained ISIL's shock recapture of the city, including its isolated location in the eastern desert of Homs province, where the group was able to overrun territory quickly.

"The geography of the city, which is surrounded by mountains, makes it very difficult to defend," said Romain Caillet, an expert on jihadist groups.