BEIRUT - Islamic State group fighters advanced to the gates of ancient Palmyra Thursday, raising fears the Syrian world heritage site could face destruction of the kind the jihadists have already wreaked in Iraq.

IS fighters were battling Syrian troops less than two kilometres (barely a mile) from the remains of one of the most important cultural centres of the ancient world, Syria's director of antiquities said.

The jihadists reached the oasis town after a lightning advance across the desert in which they overran government forces in fierce fighting that cost the lives of 110 combatants, the Syrian Observatory for Human Rights said. "Palmyra is under threat," said Observatory director Rami Abdel Rahman.

Antiquities director Mamoun Abdulkarim said he was in touch with colleagues still in Palmyra every half hour "IS has not entered the city yet, and we hope these barbarians will never enter," Abdulkarim told AFP. UNESCO describes Palmyra as a heritage site of "outstanding universal value".

The ancient city stood on a caravan route at the crossroads of several civilisations and its 1st and 2nd century temples and colonnaded streets mark a unique blend of Graeco-Roman and Persian influences.

Meanwhile, Islamic State fighters killed at least 70 government troops in a major assault by the militant group on Syrian army-held areas in the central Syrian province of Homs, and at least 40 militants also were slain, a human rights monitor said on Wednesday.

The Syrian Observatory for Human Rights, which tracks violence across the country, had earlier said 30 government fighters and 20 militants died in the fierce fighting in eastern areas of Homs province following an Islamic State assault on government forces.

Islamic State said in a statement that it had seized al Sukhna, a town some 300 km (190 miles) northeast of Damascus, in a move that gives it control over a strategic highway linking the province of Homs with Deir al-Zor province in northeastern Syria.

The hard-line fundamentalist group also claimed it had shot down a Syrian MiG jet plane in the area. The army has not commented on the developments in al-Sukhna but said it had killed scores of militants in heavy bombing of their positions in Homs and the city of Tadmr.