Beirut:
A suicide bombing that killed members of Bashar al-Assad's inner circle as fighting raged in the Syrian capital on Wednesday showed the uprising was gaining "real momentum", Washington said, urging other countries to press the president to quit.
The bomb that killed Syria's defence minister and Assad's brother-in-law will weaken morale and may accelerate high-level defections but does not signal his imminent downfall, analysts said.
"There is real momentum against Assad, with increasing defections, and a strengthened and more united opposition that is operating across the country," White House spokesman Tommy Vietor said. "We are working urgently with our international partners to push for a political transition in Syria."
British Foreign Secretary William Hague condemned the suicide attack and said it "confirms the urgent need for a Chapter 7 resolution of the UN Security Council on Syria".
Along with France, Germany and the United States, Britain has tabled a UN Security Council resolution that would extend the UN observer mission's stay in Syria for 45 days and place international envoy Kofi Annan's peace plan under Chapter 7 of the UN Charter.
Chapter 7 allows the 15-member council to authorise actions ranging from sanctions to military action.
"The situation in Syria is clearly deteriorating. All the members of the UN Security Council have a responsibility to put their weight behind the enforcement of Joint Special Envoy Kofi Annan's plan to end the violence," Hague said.
German Chancellor Angela Merkel said the bombing "shows us that it is high time to ratify the next UN resolution," and French Foreign Minister Laurent Fabius said "this degree of violence ... means that it is necessary and urgent to find a political transition that allows the Syrian people to have a government that expresses its aspirations."
Annan asked the Security Council to delay a vote planned for later on Wednesday. Russia said the vote would be postponed to Thursday.
"DECISIVE BATTLE"
With four straight days of fighting in Damascus, Moscow said the draft resolution - that it has said it would block - would only worsen the violence.
"A decisive battle is under way in Syria," Foreign Minister Sergei Lavrov told journalists in Moscow. "It is a dead-end policy to support the opposition. Assad will not go on his own and our Western partners don't know what to do about that."
Israeli Defence Minister Ehud Barak said in a video statement "We are closely and vigilantly monitoring what is going on in Syria, chiefly the possibility that this situation, with central control undermined, will lead Hezbollah to try to take advanced weapons systems from Syria.
"Vigilance on the Golan Heights will be slightly higher (than usual). But I do not think that Israel faces imminent danger," he added.
Shi'ite Muslim Iran, which is close to the Syrian leadership, members of the minority Alawite sect, an offshoot of Shi'ite Islam, blamed the "terrorist" attack in Damascus on foreign backing for the rebels.
"Sending weapons and ammunition into the country and support from some regional and international players through terrorist actions that target the robust stability and security of Syrian nation will not lead anywhere," the Foreign Ministry said in a statement reported by the Mehr news agency.
The opposition umbrella group, the Syrian National Council, said the attacks heralded a new phase in the crisis and would encourage uprisings in other areas.
"In the long run we think that this operation is the beginning of the end," SNC spokesman George Sabra said during a visit to Milan. He declined to speak in detail about who had carried out the attack.
"It is not important to talk about which individuals did the operation but the most important thing is that this activity was done by revolutionary activists and the Free Syrian Army."
EROSION OF SUPPORT
Analyst Gala Riani said the suicide bombing was "in some ways the most successful direct attack on the regime we've had so far."
"I think the next few days are going to be crucial in signalling where the conflict goes from here," said Riani, a Middle East analyst at the Control Risks consultancy.
"At the very least, we can expect the situation to continue to deteriorate. But I think it will take more than this to take the Assad regime down."
The brazen attack on a meeting of top security officials and ministers in the heart of Damascus will send a message to the top of the Syrian government that its leaders are vulnerable.
"It sends a stark message that individual ministers are not safe and is likely to accelerate the erosion of the regime's support base," said Anthony Skinner, head of Middle East consultancy Maplecroft.
The bombing, claimed by the Free Syrian Army (FSA) and also by Islamist group Liwa al-Islam, does not alter the fact that the rebels remain hugely outgunned by Assad's forces.
"These are very significant developments, but I believe the offensive will be repelled," Skinner said. "Psychologically, though, this will likely give the FSA a significant boost and may also precipitate more defections at a senior level.
"If the political and security elite become increasingly preoccupied with sustaining control of Syria's primary urban hubs, there will be fewer resources to exact revenge against the families of individuals who have bailed," Skinner said.
The fighting in the capital indicates that defectors and moles within the Assad establishment provided rebels with intelligence "making opposition forces more agile and less vulnerable to attacks," said Ayham Kamel of political risk group Eurasia.
But the offensive, which, like the bombing, could encourage more defections, may prove to be a strategic miscalculation as the rebels will be crushed by greater fire power and rebellious parts of the country could face reprisals, Kamel said.
"Heavier weapons will most likely be deployed in the next few weeks and the regime will probably move towards some form of collective punishment in rebellious regions."
© Thomson Reuters 2012