BEIRUT, Lebanon — President Bashar al-Assad’s brother-in-law and Syria’s defense minister were killed on Wednesday when a suicide bomber attacked a crisis group of senior ministers and security chiefs meeting in central Damascus, according to state television and activists.


The assassinations were the first of such high-ranking members of the power elite in the 17-month revolt against Mr. Assad’s rule, and could represent a turning point in the conflict, analysts said. The nature and target of the attack, they said, confirmed that opposition forces have been marshaling their strength to strike at the close-knit centers of state power.


President Assad’s whereabouts on Wednesday were not immediately clear.


According to state television, the dead included the defense minister, Daoud Rajha, and Asef Shawkat, the president’s brother-in-law who was the deputy chief of staff of the Syrian military. But the television report rejected claims by activists that the minister of the interior also was killed, saying he was in stable condition.


Opposition activists and Lebanese satellite channels reported later that Hassan Turkumani, a former minister of defense and military adviser to Vice President Farouk Sharaa, had died from injuries sustained in the bombing.


General Rajha was appointed minister of defense in August. A Christian, he was one of the prominent minority figures used by the Assad government to put a face of pluralism on the military and security services dominated by the president’s Alawite sect.


The British-based Syrian Observatory for Human Rights, an anti-Assad activist organization, said all the members of the crisis group set up by President Assad to try to put down the revolt were are either dead or injured. But there was no official confirmation of that account.


At the Pentagon on Wednesday morning, Defense Secretary Leon E. Panetta said that situation in 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 of the fighting," he said at a joint news conference with the British defense minister, Philip Hammond.


The attack came as diplomatic maneuvers to seek a cease-fire remained deadlocked by differences between Syria’s international adversaries and its sponsors, principally Russia, ahead of a United Nations Security Council vote scheduled later on whether to extend the mission of 300 United Nations monitors. The work of the unarmed observers has been suspended because of the violence, and they have basically been trapped in their hotel rooms since last month.


In Moscow, Deputy Foreign Minister Gennady Gatilov, offering Russia’s first official commentary on the bombing, said via his Twitter account that the attack had put consensus between members of the Security Council even farther out of reach.


“A dangerous logic: While discussions on settling the Syrian crisis are being held in the U.N. Security Council, militants intensify terrorist attacks, frustrating all attempts,” he wrote.


With tensions already high in Damascus after three days of clashes between the Syrian Army and rebels near the city center, SANA, the official news agency, described the assault as a “suicide terrorist attack” without offering any explanation of how such an assault could have been carried out in such heavily secured location. Opponents claimed a major victory.


“The Syrian regime has started to collapse,” said the activist who heads the Syrian Observatory, who goes by the pseudonym Rami Abdul-Rahman for reasons of personal safety. “There was fighting for three days inside Damascus, it was not just a gun battle, and now someone has killed or injured all these important people.”


Rumors swirled around Damascus that the bomber was the minister’s bodyguard, but there was no confirmation of those reports. The attack came despite a huge security presence to isolate embattled neighborhoods of the capital.


The casualties were from the core team trying to enforce a security solution to the uprising in Syria, and in such a tense, suspicious climate, it was not clear who Mr. Assad might find to replace them.