Hundreds more US troops heading for Syria anti-IS fight

The US is deploying an additional 400 troops for the offensive on the Islamic State group's self-proclaimed capital Raqqa in Syria, a spokesman for the US-led coalition said on Thursday.

"They are temporary forces," US Colonel John Dorrian told reporters in Baghdad.

He insisted that the number of US troops there would remain at 500, which is the country's deployment limit in Syria.

IS militants are facing simultaneous offensives in northern Syria by regime forces, Turkish-backed rebels, and a US-supported alliance of Kurdish and Arab fighters.

The US-backed Syrian Democratic Forces have battled through IS territory in Raqqa, and could reach the provincial capital's suburbs within weeks.

"We expect that within a few weeks there will be a siege of the city," SDF spokesman Talal Silo told Reuters.

A US marine artillery unit has been deployed to help the Kurdish-led forces in the battle.

SDF forces  also managed to cut a road between IS-held Syrian cities Raqqa and Deir az-Zor this week.

Agencies contributed to this story.