President Barack Obama flew into Jordan warning Syria could end up as an enclave of extremism, and pledged $200 million in fresh aid to help his host deal with the refugee crisis swamping its border.
On the final leg of a Middle East tour devoted to reassuring Israel he will deal with the threat of a nuclear Iran and to keeping long-shot Palestinian hopes of a state alive, Obama held talks with King Abdullah II in Amman.
Jordan's struggle to help 400,000 new refugees meant attention quickly focused on Obama's own policy on the bloody violence threatening to splinter Syria, and his desire to keep America out of another Middle East quagmire.
Obama admitted he was worried about what would come next in Syria, after President Bashar al-Assad is forced from power -- a scenario the US believes will eventually unfold.
"They're very good about exploiting situations that, you know, are no longer functioning. They fill that gap," Obama said.
Obama's reluctance to arm opposition groups in Syria, fearing they are, or could transform into, extremist Islamist foes with links to Al-Qaeda has dogged him on his four-day stay in the turbulent region.
On Friday, a Jordanian journalist asked him why superpower America had no plan to end the killing in Syria, prompting Obama to defend US diplomatic efforts to isolate Syria and to note hundreds of millions in US aid.
He said he would ask Congress to provide $200 million in budget support for Jordan this year as it cares for Syrian refugees.
"This will mean more humanitarian assistance and basic services, including education for Syrian children so far from home, whose lives have been upended," he said.
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, are draining its resources.
Obama warned during his visit that opened Wednesday that the use of chemical weapons by Syria's armed forces would be a game changer that would invite international action.
The United States on Friday demanded that the Syrian regime allow full access to UN weapons inspectors and cooperate with a probe into claims that chemical arms may have been used in the conflict.
"We support an investigation that pursues any and all credible allegations of possible use of chemical weapons," State Department spokeswoman Victoria Nuland told inspectors.
"And we demand the full cooperation of the Assad regime in letting the investigators in and letting them go wherever the trail leads them," she said, adding they should have "free and unfettered" access.
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The US president wrapped up his first visit to Israel as president earlier Friday by giving Prime Minister Benjamin Netanyahu, with whom he feuded in his first term, a hug.
He also pulled off an unexpected coup, engineering a deal to restore diplomatic relations between estranged US allies Israel and Turkey, concluded in a tarmac telephone call at Tel Aviv airport before he took off for Jordan.
Netanyahu apologised to Turkey and his counterpart Recep Tayyip Erdogan for a deadly raid on a Gaza aid flotilla and announced a full resumption of diplomatic ties as well as compensation to end a near three-year row.
Obama cautioned the deal, though important, should not spark too much euphoria.
"You know, this is a work in progress. It's just beginning," he said.
"There are obviously going to still be some significant disagreements between Turkey and Israel not just on the Palestinian question but on a range of different issues."
The US leader wrapped up a three-day trip to Israel and the Palestinian territories, his first as president with a visit to Bethlehem's Church of the Nativity.
He also visited the grave of Theodor Herzl, the father of modern Zionism, then paid his respects at the grave of murdered Israeli premier Yitzhak Rabin, where he placed a stone from the grounds of Washington's Martin Luther King memorial.
Touring Yad Vashem Holocaust Museum, he said the haunting site showed the barbarism "that unfolds when we begin to see our fellow human beings as somehow less than us."
After a brief meeting with Netanyahu, Obama set off for Bethlehem, travelling by motorcade, not helicopter, after a sudden sandstorm swept the city.
The change of plan gave Obama an unscheduled experience of the eight-metre-tall (26-foot) wall which loops around the West Bank city.
As the huge motorcade wound through the steep, narrow streets, crowds of onlookers watched in silence, with no sign of the enthusiasm which usually greets the convoy.
Some held up signs of protest reading: "No return no peace."
Inside the cavernous, dark interior of the church, he was briefly shown around by Palestinian president Mahmud Abbas, then had his picture taken with a group of children waving US and Palestinian flags.
In a powerful direct appeal to young Israelis on Thursday, Obama declared the two-state peace solution was very much alive and their only hope of true security, urging them to try and see the situation through Palestinian eyes.