BEIRUT, Lebanon — 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. In a move to dispel any rumors that he had been injured or had left the capital, Mr. Assad appeared on state television on Thursday swearing in a new defense minister in what appeared to be a reception room at the presidential palace. The images were broadcast in a continuous loop on SANA.

The outlook for a peaceful outcome to the conflict 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.