Islamic State group jihadists battled Syrian troops near ancient Palmyra Thursday, threatening the UNESCO world heritage site with destruction like that which it has already wreaked in Iraq, a monitoring group said.

After a lightning advance across the desert in which they overran government forces in ferocious fighting in which 110 combatants were killed, the jihadists were within two kilometres (a little more than a mile) of the ruins, the Syrian Observatory for Human Rights said.

"Palmyra is under threat," said Observatory director Rami Abdel Rahman.

"IS has taken all the army posts between Al-Sukhnah and Palmyra" in its drive towards the oasis town from its stronghold in the Euphrates valley to the east, he added.

UNESCO describes Palmyra as a heritage site of "outstanding universal value". It contains the monumental ruins of a great city that was one of the most important cultural centres of the ancient world.

The threat to its well-preserved 1st and 2nd Century temples and colonnaded streets comes as an international conference is under way in Cairo to address the destruction already wreaked by IS on the ancient sites of Nimrud and Hatra in Iraq. (*)