BEIRUT (AFP) - A Russian aerial onslaught forced ISIS fighters to withdraw from Syria's Palmyra at dawn on Sunday (Dec 11), only hours after the militants had re-entered the ancient city, a monitor said.

"Intense Russian raids since last night forced IS out of Palmyra, hours after the jihadists retook control of the city," said Rami Abdel Rahman of the Syrian Observatory for Human Rights monitoring group, referring to the ISIS militants.

The raids killed a large number of the militants in the desert city in central Syria, Abdel Rahman said, without being able to give a precise number.

"The army brought reinforcements into Palmyra last night, and the raids are continuing on jihadist positions around the city," Abdel Rahman told AFP.

ISIS began an offensive just days ago near Palmyra, which is on Unesco's World Heritage list.

In May last year, the Sunni Muslim extremist group seized several towns in Homs province including Palmyra, where they caused extensive damage to many of its ancient sites.

They were ousted from Palmyra in March by Syrian regime forces backed by Russia.