The U.S.-backed Syrian Democratic Forces penetrated the Islamic State group's bastion of Tabqa in northern Syria on Monday, overrunning territory in the town's south, a monitor said.

 

"The SDF entered Tabqa for the first time after besieging it from all sides," the Syrian Observatory for Human Rights said.

 

"They seized control of several points in the town's south and were advancing on its western edges," said Observatory head Rami Abdel Rahman.

 

In an online statement, the SDF said it had captured IS-held positions in west Tabqa, including a roundabout, and part of a southern district.

 

"There are now clearing operations in the liberated positions," the SDF said.

 

Tabqa and the nearby major dam of the same name are considered key prizes in a broader offensive for Raqa, the de facto Syrian capital of IS' self-proclaimed "caliphate", 55 kilometers (34 miles) to the east.

 

IS is under pressure on several fronts, with government forces attacking it elsewhere in Syria and a U.S.-backed offensive targeting its Iraqi stronghold of Mosul.

 

The SDF launched its campaign for Raqa in November and has since captured most of the surrounding province.