NEW DELHI: US Secretary of State John Kerry headed Tuesday to Saudi Arabia in hopes of coordinating support for Syria's rebels amid fears that a prolonged civil war will embolden extremists.

Kerry will spend several hours in the western city of Jeddah consulting with the leadership of the oil-rich Sunni Muslim monarchy, which has been outspoken in its opposition to embattled Syrian President Bashar al-Assad.

President Barack Obama is cautious about deeper US involvement in the increasingly sectarian conflict but has vowed to step up support for the rebels after concluding that Assad defied warnings and used chemical weapons.

US policymakers have privately voiced concern that Saudi Arabia and fellow monarchy Qatar could increasingly embrace hardline Sunni guerrillas in strategically placed Syria if Western powers leave a vacuum.

Kerry told a conference of major powers in Qatar on Saturday that the United States and its partners were boosting military aid to Syria's mainstream rebels, although he has been reticent on the exact nature of the assistance.

In interviews ahead of his departure to Jeddah, Kerry declined to raise concerns about potential Saudi and Qatari support for hardliners but said it was critical to strengthen moderate rebels to prevent an Assad victory.

"If the United States does nothing, and the rest of the world does nothing, then Syria is going to wind up in an even worse condition than it is today," Kerry told CBS News from New Delhi.

A worse scenario in Syria could include "a total breakup, with radicals, extremists able to get a hold of chemical weapons and free to use it as a base to begin to conduct their operations again against the West and the United States," Kerry said.

Saudi Arabia, while a longstanding US ally, practises a puritanical form of Wahhabi Islam. US officials have in the past voiced concern about money from Gulf Arabs funding Sunni hardliners in Afghanistan, Pakistan and elsewhere.