AMMAN: U.S. President Barack Obama and Jordan's King Abdullah presented a united front against Syrian President Bashar al-Assad on Friday as Jordan grapples with a refugee crisis caused by Syria's civil war.

Obama, in Jordan following a visit to Israel and the West Bank, pledged new aid to help the country deal with the refugee crush but stopped short of promising military assistance to Syrian rebels to speed the departure of Assad after a two-year civil war that has claimed 70,000 lives.

Syria dominated talks between Obama and King Abdullah. Jordanian authorities worry that any emergence of Islamist rule in a post-Assad Syria could embolden Islamists who are the main opposition group in Jordan.

Obama pledged to work with the U.S. Congress to provide $200 million in extra assistance to care for Syrian refugees who now number 460,000 in Jordan, a figure that the king said was equal to 10 percent of Jordan's population and may double by year's end.

King Abdullah vowed he would not close Jordan's borders to the refugees, calling it "a challenge that we just can't turn our backs on".

Obama, who has wound down U.S. involvement in wars in Iraq and Afghanistan, demonstrated a wariness about Syrian rebels even as he warned that Syria could become an enclave of extremism if a political transition does not take place.

"Ultimately what the people of Syria are looking for is not replacing oppression with a new form of oppression," he said.

"I'm confident that Assad will go," said Obama. "It's not a question of if, but when."

Obama came to Jordan to reassure King Abdullah of Washington's support as the country tries to cope with the influx of refugees and economic difficulties.

Welcomed at an elaborate ceremony at the king's palace that included soldiers riding camels, Obama gently encouraged Abdullah to stay on the path of what he called "necessary" political and economic reforms.

Jordan has been the scene of mostly peaceful street protests, rather than the uprisings that have shaken some of its neighbours, and the king has responded with cautious steps toward democracy.

Obama needs Abdullah's help to advance peace prospects between Israel and the Palestinians. While in Israel and Ramallah, Obama attempted to foster a dialogue between the parties.

"My hope and expectation is that as a consequence of us doing our homework, we can explore with the parties a mechanism for them to sit back down, to get rid of some of the old assumptions and to get this done," said Obama.

Jordan is one of only two Arab states - Egypt is the other - to have signed peace treaties with Israel, and is seen as a potential player in any future U.S.-led peace push. It also has a majority Palestinian population.

Abdullah offered to play host to Israelis and Palestinians for talks if that is what they want. He said the window of opportunity was fast closing due to continued Israeli settlement activity on the West Bank.