Washington will send another 200 troops to Syria to help an alliance of Kurdish and Arab fighters seize the Islamic State group bastion of Raqa, Defence Secretary Ashton Carter said on Saturday.

"I can tell you today that the United States will deploy approximately 200 additional US forces in Syria," Carter told Gulf policymakers in the Bahraini capital Manama.

They will complement 300 American special forces already in Syria to assist US-backed Kurdish-Arab troops who in recent weeks began their offensive on Raqa.

That operation coincides with a US-backed Iraqi effort to retake Mosul.

The two cities are the last major urban centres under IS control after the jihadists suffered a string of territorial losses in Iraq and Syria over the past year.

Carter told the Manama Dialogue security forum that the troop reinforcements will include bomb disposal experts and trainers as well as special forces.

Car bombs and elaborate networks of booby traps and mines have been the jihadists' favoured weapons as they have battled to defend what remains of the "caliphate" they declared across Iraq and Syria in 2012.

Source: AFP