US President Barack Obama said in Jordan on Friday that he was "very concerned" that neighbouring war-torn Syria could become an enclave for extremism.
"I am very concerned about Syria becoming an enclave for extremism because extremists thrive in chaos, they thrive in failed states, they thrive in power vacuums," he told a joint news conference in Amman with King Abdullah II.
Fears that extremists may seize more power in Syria have prompted US caution on the spiralling conflict.
Washington believes that one of the strongest Syrian opposition militias, Al- Nusra Front, is a terrorist organisation that is indistinguishable from the group Al-Qaeda in Iraq.
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Obama also announced that he would ask the US Congress to provide $200 million in "budget support" for Jordan to provide for the refugees who the monarch said now numbered more than 460,000.
The American president, who arrived in Jordan to face scrutiny over his Syria strategy, said the funds would help cash-strapped Jordan provide more humanitarian services to Syrian refugees.
He said it was "heart-breaking" to see the suffering of Syrian children.
"It's heart-breaking for any parent to see children going through tumult," he told reporters.
At least 120,000 Syrian refugees are in the sprawling northern border camp of Zaatari alone, and Jordan has repeatedly complained that the growing numbers of Syrians, expected to reach 700,000 this year, is draining its already limited resources.