The United States has said 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.
"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," said Jay Carney, the White House spokesman, to reporters in Wednesday.
President Barack Obama called Russian President Vladimir Putin, Assad's main international supporter, after a Damascus bomb blast reportedly killed Syria's defense minister and Assad's brother-in-law, throwing the 16-month old rebellion onto an unpredictable path.
The White House said Obama discussed the deteriorating Syrian situation with Putin, whose government has repeatedly blocked efforts to rally the UN Security Council behind tough measures against Damascus.
But while the two leaders agreed on the need to stop the violence, both Russian and US 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 opposed to what it sees as a Western-led attempt to dictate the outcome of Syria's crisis, the UN Security Council delayed until Thursday a proposed vote on a US-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.
The White House, reiterating the US 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 emphasised that Assad's government would be held responsible if it failed to safeguard its chemical weapons sites.
"This is a situation that is rapidly spinning out of control," Panetta told a Pentagon news conference.
US officials said they did not have any information on Assad's whereabouts, and declined to speculate on who might have been behind the attack.
Pushing ahead with unilateral sanctions, the US Treasury on Wednesday added 29 Syrian officials including virtually all of Assad's cabinet to the official US blacklist.
The US 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 a 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.
Officials said they had no indication Syria had lost control of any of its chemical weapons, but Panetta warned that the US 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.