BEIRUT: Islamic State fighters in Syria have entered the ruins of Palmyra after taking complete control of the historic city, a group monitoring the war said on Thursday, putting the world heritage site at risk of destruction.

The Al Qaeda offshoot controlled the central city's military air base, prison and intelligence headquarters, having stormed it on Wednesday, said Rami Abdulrahman, head of the Syrian Observatory for Human Rights.

Clashes since Wednesday had killed at least 100 pro-government fighters, he said. Pro-government forces have withdrawn, according to Syrian state media.

Abdulrahman, who bases his information on a network of sources on the ground, said there were so far no reports of destruction of the city's historic artifacts.