WASHINGTON 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.
The White House, reiterating the U.S. view that Assad's days are numbered, said the international community must now come together around a political transition plan to establish a democratic order in Syria after Assad departs.
"The sooner this transition happens, the greater the chance we have of averting a lengthy and bloody sectarian civil war and the better we'll be able to help Syrians manage a stable transition to democracy," said Tommy Vietor, another White House spokesman.
Defense Secretary Leon Panetta, speaking earlier, said Syria was lurching into dangerous territory and emphasized that Assad's government would be held responsible if it failed to safeguard its chemical weapons sites - a major concern for U.S. and regional security planners.
"This is a situation that is rapidly spinning out of control," Panetta told a Pentagon news conference.
One European national security official, reflecting the intelligence estimate of Washington and its allies, said that while the deadly bomb blast that wiped out key Syrian security officials was a huge setback for Assad, it was not necessarily a death blow.
U.S. officials said they were still assessing reports of the Damascus attack - but did not condemn it outright.
"The United States does not welcome further bloodshed in Syria. We note, however, that these men were key architects of the Assad regime's assault on the Syrian people," State Department spokesman Patrick Ventrell said.
U.S. officials said they did not have any information on Assad's whereabouts, and declined to speculate on who might have been behind the attack.
"I have seen nothing yet that would lead me to believe we have a clear determination of who was behind it. It certainly has all the elements of an al Qaeda-type attack, knowing that al Qaeda has a significant presence there," Rep. Mike Rogers, the Republican chairman of the U.S. House of Representatives intelligence committee, told Reuters.
"When there is confusion in the capital city all of these groups are going to try to take advantage of it."
U.S. officials emphasized that the chaos in Syria underlined the need for U.N. action, and said they were pressing the Security Council to support international mediator Kofi Annan's call for a resolution, which spells out consequences for Assad's failure to implement a widely ignored peace plan.
But the five permanent veto-wielding council members - the United States, Russia, Britain, China and France - remain deeply divided and U.S. officials indicated there was little progress thus far.
"We had a useful discussion but I wouldn't call it a negotiation," U.S. Ambassador Susan Rice told reporters after further talks between the permanent five members on Wednesday.
Pushing ahead with unilateral sanctions, the U.S. Treasury on Wednesday added 29 Syrian officials including virtually all of Assad's cabinet to the official U.S. blacklist.
The United States also designated one company controlled by Rami Makhluf, who the Treasury statement called a "crony" of Assad, as well as five companies linked to the Syrian government agency responsible for non-conventional weapons programs.
Syria's undeclared chemical weapons stockpile - believed to be the largest of its kind in the Middle East - has been reported to include sarin nerve agent, mustard gas and cyanide.
Western and Israeli officials have said in recent days that the Assad government appears to be shifting some chemical weapons from storage sites, but it is not clear whether the operation is merely a security precaution.
The Syrian government denies carrying out the operation.
U.S. officials said they had no indication Syria had lost control of any of its chemical weapons, but Panetta warned that the United States would be watching closely.
"We've made very clear to them that they have a responsibility to safeguard their chemical sites and that we will hold them responsible should anything happen with regards to those sites," Panetta said.
(Additional reporting by David Alexander, Phil Stewart, Tabassum Zakaria, Mark Hosenball, Margaret Chadbourne; editing by Vicki Allen and Todd Eastham)