The United States will send 200 additional military personnel including special forces to the campaign against Islamic State in Syria to create a "tornado" of pressure against the group's Raqqa hub, U.S. Defence Secretary Ash Carter said today.

Carter, speaking in Bahrain to regional security chiefs, twinned the announcement with a call on Middle East allies to do more for their own defence, a sore topic with some Gulf states who resent being seen by Washington as military "free riders".

The arrival of the 200 additional forces in Syria, joining 300 special forces already there backing local allies, would bring to bear the "full weight of U.S. forces around the theatre of operations like the funnel of a giant tornado", Carter said at the Manama Dialogue security conference.

Syria's civil war pits Assad, backed by Iran, Russia and some Shi'ite militias, against mostly Sunni Arab rebels backed by Turkey, Gulf monarchies and the United States.

A secondary conflict puts all of them at war with Islamic State.

Mosul in Iraq and Raqqa in Syria are the pillars of Islamic State's self-declared caliphate, and recapturing them would be a pivotal defeat for the ultra-hardline Sunni jihadists.

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