Jihadists from the Islamic State (IS) group seized control of the northern part of Syria's ancient desert city of Palmyra on Saturday after fierce clashes with government forces, a monitoring group said.
"IS 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 jihadist 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 southwest of the city.
IS 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.
Rahman said family members of government employees were among those murdered.
The world heritage site lies at the gateway to Syria's vast desert, extending to IS-controlled territory in neighbouring Iraq.
There are fears that IS would see Palmyra's cultural treasures as targets, after the group was filmed destroying the Iraqi archaeological sites of Nimrud and Hatra.
IS has been battling Syrian government troops in and around the city, amid heavy shellfire and regime bombardment by air, the Syrian Observatory for Human Rights reported.
Government positions to Palmyra's southwest -- where the artefacts and ruins are located -- held firm at one kilometre (less than a mile) from the city's old Islamic citadel.
If Palmyra falls, IS would expand its control from Syria's east and north to the country's heartland.
The city is also key to IS's propaganda machine, as the group has destroyed numerous pre-Islamic archeological sites during its rampage across Iraq and Syria.
Fearing a similar fate for Palmyra, known as the "pearl of the desert," Unesco has called on the UN Security Council to act in order to save one of West Asia's historic treasures.
Both Syria's opposition and regime have demanded that the renowned site not fall to the jihadists.