The New York Times reports: Fighting seized neighborhoods encircling Damascus for a fifth straight day on Thursday, a day after President Bashar al-Assad’s key security aides were killed in a brazen bombing attack in the sharply escalating conflict.

The bombing, 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 outlook for a peaceful outcome in Syria darkened further on Thursday, when Russia and China vetoed a Britain-sponsored resolution at the United Nations Security Council that would have penalized Mr. Assad’s government with sanctions for the first time for failing to implement the six-point peace plan negotiated by Kofi Annan, the special Syria envoy. The double veto also called into question the viability of a 300-member United Nations mission sent to Syria to monitor the peace plan. Its mandate expires Friday.

Opposition activists reported battles between the Army and opposition forces in the southern district of Damascus and in the northern suburb of Qaboun, with residents who were not trapped by fighting fleeing many areas. In a second statement in two days, the Syrian military said on Thursday that the bombing had left it more determined to “clear the homeland of the armed terrorist groups” — the term it uses for the insurgents seeking Mr. Assad’s ouster.

The Syrian Observatory for Human Rights, based in Britain, said that the government assault had intensified, with more helicopters firing rockets that were igniting and destroying houses in Qaboun. It said that snipers were deployed around the exits of the neighborhood and that water and electricity had been cut off with numerous families trapped and no one able to excavate dead bodies from the rubble.

One activist reached in Damascus, using only the name Omar, said that the government had been asking residents of Tadamon and parts of Yarmouk, the capital’s southern neighborhoods, to leave their homes. That is usually a sign that government forces are on the verge of a violent attack.

Residents of Mezze and Kafr Sousseh, western neighborhoods even closer to the center of the city, fled unprompted because of the intensity of the shelling there, activists said.