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WASHINGTON (Reuters) – The United States said on Wednesday that Syrian President Bashar al-Assad was losing control of his country and urged Russia and the international community to get behind a political transition plan to avert sectarian civil war.

U.S. President Barack Obama called Russian President Vladimir Putin – Assad’s main international supporter – after a Damascus bomb blast killed Syria’s defense minister and Assad’s brother-in-law, throwing the 16-month old rebellion onto an unpredictable new path.

“The window is closing, we need to take action in a unified way to help bring about the transition that the Syrian people so deserve,” White House spokesman Jay Carney said at a news conference.

The White House said Obama discussed the deteriorating Syrian situation with Putin, whose government has repeatedly blocked efforts to rally the U.N. Security Council behind tough measures against Damascus.

But while the two leaders agreed on the need to stop the violence, both Russian and U.S. officials said they ended the call divided over the best way forward.

“They noted the differences our governments have had on Syria, but agreed to have their teams continue to work toward a solution,” the White House said in a statement.

With Russia adamantly opposed to what it sees as a Western-led attempt to dictate the outcome of Syria’s crisis, the U.N. Security Council delayed until Thursday a proposed vote on a U.S.-backed resolution that threatens Damascus with sanctions if it does not stop using heavy weapons and withdraw troops from towns and cities.

Russia has indicated it will likely veto the measure if it comes to a vote.