US-backed forces battled Islamic State group fighters around a key northern Syrian town on Monday, a monitor said, hours after seizing a military airport from the jihadists. 

 

The capture of Tabqa airbase late Sunday comes as the Syrian Democratic Forces (SDF) prepare an attack on IS's de facto Syrian capital Raqa, seeking to surround the city before launching their assault.

 

On Monday morning, SDF fighters pressed their offensive around the base, according to the Syrian Observatory for Human Rights monitoring group. 

 

"There is fighting north of airport as the SDF tries to reinforce its positions there," Observatory head Rami Abdel Rahman said, adding that there were "heavy air strikes" in the area. 

 

The base is approximately 2.7 kilometres (less than two miles) south of the town of Tabqa, an important IS stronghold on the southern bank of the Euphrates River.

 

Backed by air power from the US-led coalition bombing IS, the SDF's Arab and Kurdish units are approaching the town from the south via the airport, and from the north near the Tabqa dam. 

 

Abdel Rahman said the SDF was also locked in fierce clashes on Monday near the dam's northeastern entrance. 

 

The Tabqa dam, still held by IS, was forced out of service on Sunday after its power station was damaged, a source there told AFP. 

 

The UN has warned that damage to the dam "could lead to massive scale flooding across Raqa and as far away as Deir Ezzor" province to the southeast.

 

"We will begin rehabilitating the airport after clearing out explosive devices" left behind by IS, SDF spokesman Talal Sello told AFP, describing heavy damage to the base's main landing strip. 

 

He said SDF fighters would continue fighting around the dam and towards the town of Tabqa "in order to complete the siege of Raqa."

 

The SDF launched their offensive for Raqa -- the de facto Syrian capital of IS's "caliphate" -- in November. 

 

At their closest point, they are just eight kilometres (five miles) from the city, to the northeast.

 

But they are mostly further away, between 18 and 29 kilometres from Raqa.