BEIRUT, Lebanon — Widespread violence erupted in Damascus on Wednesday as Syrian security forces and pro-government militias lashed out in revenge for a bombing that killed at least three crucial figures in the nation's military establishment, calling into question President Bashar Assad's control even over his capital.

The blast targeted a meeting of the top security chiefs charged with overseeing a crackdown against the country's 16-month revolt. The bombing suggested the rebels have penetrated the most loyal core of Assad's inner circle of advisers.

The dead included Defense Minister Dawoud Rajha; Hassan Turkmani, a former minister of defense who headed the government's crisis-management cell; and Assef Shawkat, Assad's brother-in-law and deputy chief of staff of the Syrian military.

The government denied reports that other top figures also were killed in the bombing at the National Security Building in the heart of one of the capital's most upscale and closely guarded neighborhoods. But the significance of the identities of those confirmed dead was not lost on Syrians or the international community.

"It's obvious that what's happening in Syria represents a real escalation in the fighting," Defense Secretary Leon Panetta said at a Pentagon briefing. "This is a situation that is rapidly spinning out of control."

The rebel Free Syrian Army said its loyalists planted bombs inside a room where the government's central command unit for crisis management — a special cell made up of about a dozen top security chiefs — was meeting to discuss efforts to crush the uprising.

The bombs were detonated remotely once the meeting began, said Col. Malik Kurdi, the rebel group's deputy commander. "The Free Syrian Army carried out this attack in retaliation for the massacres committed by the regime and because of the international silence," Kurdi said. "We promised that we are going to hit the regime in its most sensitive axis. This was necessary for us."

The government said others were injured. Some news outlets reported Interior Minister Mohammed Shaar was seriously hurt and eventually died, but the state-run Syrian Arab News Agency said he and another official identified only as Lt. Gen. Hisham were in "stable" condition. The agency apparently was referring to Hisham Ikhtiar, Assad's national-security chief.

President Obama and Russian President Vladimir Putin discussed the deteriorating situation in Syria, an administration spokesman said. . Obama cautioned Putin that maintaining Russia's alliance with the Assad government would put his country on the "wrong side of history," administration spokesman Jay Carney said.

At the Pentagon, Panetta and visiting British Defense Secretary Philip Hammond said the violence illustrates the need for Assad to cede power peacefully, under a transition plan brokered by U.N.-Arab League special envoy Kofi Annan.

In London, British Foreign Secretary William Hague condemned the bombing and called on all members of the U.N. Security Council, including Syrian allies Russia and China, to pressure Assad to accept the Annan plan.

But Russian Foreign Minister Sergei Lavrov dismissed such demands, saying they were "rooted in hopelessness."

The U.N. Security Council delayed a vote on a new Syria resolution until Thursday in a last-minute effort to persuade key Western nations and Russia to reach agreement on measures to end the violence, The Associated Press reported.

Annan urged the council to delay Wednesday's scheduled vote after the Damascus bombing, AP said.

That the bomber penetrated so deeply into the heart of the establishment could have a powerful effect on morale, within Assad's Cabinet and across the ranks of the military and government supporters who have remained loyal.

Within hours, fresh defections from security services were reported, as well as revenge attacks by Assad loyalists in Damascus.

Damascus residents reported that pro-government militiamen known as shabiha were swarming into several neighborhoods, bent on vengeance. According to one witness in the Shaghour neighborhood of the historic walled Old City, militiamen were breaking down doors and killing families with knives. The man said he was watching scenes of panic from the roof of his building, as men and children, carrying guns and knives, ran into the street to try to defend the area.

Opposition activist Tareq Saleh of the Revolutionary Leadership Council of Damascus said he was receiving reports of similar killings in the areas of Hajar al-Aswad and Qadam.

"Shabiha militias are killing people with knives," he said. "There are tens of bodies on the streets."

The information was impossible to corroborate independently, but activists posted videos of what appeared to be three bodies in the streets of Qadam.

Elsewhere in the capital, streets were calm but tense, with most shops closed and few people venturing out. "There is so much fear among the people," Saleh said. "We are expecting massacres."

There were numerous reports that many army soldiers had defected in the northern province of Idlib and in parts of the flash point city of Homs.

According to a Homs activist who calls himself Abu Emad, more than 250 soldiers were seen abandoning the Old City neighborhood, long a battleground between rebels and the government.

At the same time, Assad loyalists still in position around the edge of the city were shelling the area with renewed intensity, he said.

Hours after Wednesday's bombing, the Local Coordination Committees, an opposition group, reported heavy clashes between the military and rebel forces in Qaboun. They said helicopters were being used. Shelling also was reported in the capital's Midan neighborhood.

The bombing, which sent a huge plume of smoke over the Damascus skyline, was planned over the past two months, said Kurdi, the Free Syrian Army officer. He said the rebels had information about the crisis group's meetings and were monitoring movements of the senior officials. Kurdi also said there was an earlier plan to poison food served at these meetings, but that plan fell through in May.

A separate claim of responsibility for the bombing came from a little-known group calling itself the Brigade of Islam, which implied the strike was a suicide attack. The claim could not be verified.

The death of Shawkat, married to Assad's elder sister, Bushra, was especially significant because of his standing both as an Assad family member and as a key figure in the effort to crush the uprising, said Amr al-Azm, a history professor at Ohio's Shawnee State University who is also active in the Syrian opposition.

"Assef Shawkat was not only a very close member of the Assad family but also a forceful and powerful member of the inner decision-making circle," Azm said. "He was well-known for being brutal, effective and decisive, and he was at the forefront of the fight against the uprising."