US-backed Syrian force starts battle to capture Raqqa from IS

The US-backed Syrian Democratic Forces (SDF) said on Tuesday it had begun a battle to capture Raqqa, IS's de facto capital in Syria, launching attacks from the east, west and north of the city.

Fighters from the SDF running during fighting with IS militants, in Raqqa's eastern countryside, Syria. Source: AAP

In a phone interview with Reuters from Syria, SDF spokesman Talal Silo said the operation started on Monday and the fighting would be "fierce because Daesh (IS) will die to defend their so-called capital".

IS captured the city from rebel groups in 2014 and has used it as an operations base to plan attacks in the West.

The assault on Raqqa will pile more pressure on IS's self-declared "caliphate" with the group facing defeat in the Iraqi city of Mosul and being forced into retreat across much of Syria.

"The coalition has a big role in the success of the operations. In addition to warplanes, there are coalition forces working side by side with the Syrian Democratic Forces," Silo said.

The Syrian Observatory for Human Rights, a Britain-based organisation that reports on the war, earlier said the SDF attacked the eastern edge of Raqqa and a military base on the northern outskirts of the city on Tuesday.

The Kurdish YPG, part of the SDF, told Reuters on Saturday the assault on Raqqa was expected to start in a few days.

"It started today at dawn," Observatory Director Rami Abdulrahman said. "They have reached the city but they have not entered any of its buildings."

Watch: IS targets in Syria's Al Sukhnah hit by Russian airstrikes

The attack on the al-Mashlab district and on the Division 17 base around 1 km to the north of the city centre followed heavy overnight air strikes, the Observatory said.

The SDF has been working to encircle Raqqa since November in an offensive backed by the US-led coalition that is also fighting IS in Iraq, where the jihadist group is on the brink of defeat in the city of Mosul.

The U.S.-led coalition has said 3000 to 4000 IS fighters are thought to be holed up in Raqqa city, where they  have erected defenses against the anticipated assault. The city is about 90 km from the border with Turkey.

The United States said on Tuesday it had started distributing arms to the YPG to help take Raqqa, part of a plan that has angered NATO-ally Turkey, which is worried by growing Kurdish influence in northern Syria.