BEIRUT (AFP) - Militants from the Islamic State in Iraq and Syria (ISIS) group seized control Saturday of the northern part of Syria's ancient desert city of Palmyra after fierce clashes with government forces, a monitoring group said.

"ISIS advanced and took control of most of northern Palmyra, and there are fierce clashes happening now," said Rami Abdel Rahman, director of the Syrian Observatory for Human Rights.

He said 13 extremist fighters were killed in ongoing clashes near the Islamic citadel in the city's west.

Abdel Rahman had no details on regime casualties.

Most of Palmyra's renowned ruins, including colonnaded streets and elaborately decorated tombs, lie to the south-west of the city.

The official Syrian news agency Sana quoted a miliary source as saying that regime forces had prevented IS fighters from seizing a hilltop south-west of the Islamic citadel.

The head of Syria's antiquities department, Mamoum Abdulkarim, meanwhile, voiced concern for the ancient site.

"I am living in a state of terror," Abdulkarim told AFP in a telephone call.

He said ISIS "will blow everything up. They will destroy everything," if they enter the site, adding that many of Palmyra's artefacts, like elaborate tombs, could not be moved.

"If they enter the ancient ruins, it will be worse than when Palmyra was defeated in the time of Zenobia," said Abdulkarim.

Zenobia ruled over Palmyra - a Unesco world heritage site - as queen in the third century, but was dramatically defeated by the Romans.

ISIS began its offensive on Palmyra on Wednesday and inched closer to the ancient metropolis on Thursday and Friday, executing at least 49 civilians over those two days according to the Observatory.