Syria Clashes Continue After Bombing Kills Key Assad Aides Smoke billowed over Damascus, Syria, on Wednesday. BEIRUT, Lebanon - The killing on Wednesday of President Bashar al-Assad's key security aides in a brazen bombing attack, close to Mr. Assad's own residence, called into question the ability of a government that depends on an insular group of loyalists to function effectively as it battles a strengthening opposition. The strike dealt a potent blow to the government, as much for where it took place as for the individuals who were targeted: the very cabinet ministers and intelligence chiefs who have coordinated the government's iron-fisted approach to the uprising. The defense minister and the president's brother-in-law were both killed, and others were seriously wounded. Activist groups opposed to Mr. Assad said on Thursday that fighting had continued overnight and into the morning in Damascus, with government forces shelling some neighborhoods and residents fleeing the onslaught. The attack on the leadership's inner sanctum as fighting raged in sections of the city suggested that the uprising had reached a decisive moment in the overall struggle for Syria. The battle for the capital, the center of Assad family power, appears to have begun. Though there was no indication he was wounded, Mr. Assad remained out of public view on Wednesday and Thursday - unusual but not unprecedented in a secretive country where the government has long tried to present an image of quiet control. In Washington, Defense Secretary Leon E. Panetta said that Syria "is rapidly spinning out of control," and warned Mr. Assad's government to safeguard its large stockpile of chemical weapons. "It's obvious what is happening in Syria is a real escalation," he said at a joint news conference with the British defense minister, Philip Hammond. The attack drew dire assessments elsewhere. "It pains me to say, but we are not on the track for peace in Syria," Maj. Gen. Robert Mood, the Norwegian head of the United Nations monitoring mission in Syria told reporters in Damascus a day before the group's mandate expires, "and the escalations we have witnessed in Damascus over the past few days is a testimony to that." For the sake of the Syrian people, we need effective leadership from the Security Council and genuine unity around a political plan that meets the aspirations of the Syrian people and that is accepted by the parties. Government and opposition must be willing to make the necessary concessions and sit at the negotiating table," he said. During a visit to Kabul, Afghanistan, on Thursday, the British prime minister, David Cameron, called for Mr. Assad's ouster and for the support of Russia - Syria's main international sponsor - in imposing "tough" sanctions, a step Moscow has strongly resisted. "It is time for him to go, it is time for transition in this regime," Mr. Cameron said. The sooner that happens, the sooner the people of Syria can be freed from the tyranny under which they are suffering. Clearly, Britain doesn't support violence on either side, but if there isn't transition there is going to be civil war. That is the clear fact that we can all see on the ground," he said.In Syria, the impact of Wednesday's events reverberated on multiple levels, piercing the psychological advantage that Mr. Assad's superior military strength has provided in preserving the loyalty of his forces and frightening much of the public into staying home. With the opposition energized and the government demoralized, analysts wondered if other military units and trusted lieutenants would be more inclined to switch sides - and if the government would retaliate with an escalation of violence. The idea that a poorly organized, lightly armed opposition force could somehow get so close to the seat of power raised questions about the viability of a once unassailable police state. The Assad family has for decades relied on overlapping security forces and secret police to preserve its lock on power. At best, for Mr. Assad, the system failed. At worst, for Mr. Assad, defectors or turncoats helped carry out an inside operation. The government said that the attack was the work of a suicide bomber, while an officer with the Free Syrian Army said it was a remotely detonated explosive. Reporting was contributed by Dalal Mawad and Hwaida Saad from Beirut, Alan Cowell from London, Matthew Rosenberg from Kabul, Afghanistan, Rick Gladstone from New York, Ellen Barry from Moscow, Isabel Kershner from Jerusalem, Elisabeth Bumiller and Eric Schmitt from Washington, and an employee of The New York Times from Damascus, Syria.