Sunni militants have advanced in western Iraq and killed 21 people after security forces withdrew from several towns, as US President Barack Obama warned the offensive could spill over into other regional nations.

The militants, led by the jihadist Islamic State of Iraq and the Levant (ISIL), seized the towns of Rawa and Ana after taking the Al-Qaim border crossing on Saturday, residents said.

ISIL aims to create an Islamic state incorporating both Iraq and Syria, where the group has become a major force in the rebellion against President Bashar al-Assad.

Washington wants Arab states to bring pressure on Iraq's leaders to speed up government formation, which has made little headway since April elections, and has tried to convince them ISIL poses as much of a threat to them as to Iraq.

Washington backed Maliki when he first became premier in 2006, as he was seen to be cracking down on Shiite militias while reaching out to Sunni leaders.

Obama has offered to send up to 300 military advisers to Iraq, but has so far not backed air strikes as requested by Baghdad.

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