A file picture taken on March 14, 2014 shows the citadel (background) of the ancient oasis city of Palmyra, 215 kilometres northeast of Damascus, over looking the city.

More than half of Syria’s territory is now controlled by the Islamic State of Iraq and Syria group after its westwards advance into the ancient city of Palmyra, a group monitoring the war said on Thursday.

The Britain-based Syrian Observatory for Human Rights told Reuters the militant group has taken control of the city’s military air base and prison after storming them on Wednesday.

ISIS followers posted a statement on Twitter saying it was in full control of the city, including the military installations, and that the retreating pro-government forces had left behind a large number of their dead.

Clashes since Wednesday had killed at least 100 pro-government fighters, the observatory said, which gains information through a network of sources on the ground.

The capture of Palmyra is the first time ISIS has taken control of a city directly from the Syrian army and allied forces, which have already lost ground in the northwest and south to other insurgent groups in recent weeks.

The central city, also known as Tadmur, is built alongside the remains of an oasis civilization whose colonnaded streets, temple and theatre have stood for 2,000 years

The areas ISIS holds are mostly sparsely inhabited. Syria’s main cities, including the capital Damascus, are located on its western flank along the border with Lebanon and the coastline and have been the priority for the Syrian military.

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