Russia and China have vetoed a UN Security Council resolution that would have imposed further sanctions on Syria's government if it continued using heavy weapons.
Ahead of the vote, Russia had said the resolution was unacceptable because it could ultimately be enforced by military action.
The veto on Thursday is the third by Russia and China on UN resolutions designed to put pressure on President Assad.
Meanwhile, Syrian state television has broadcast pictures of President Bashar al-Assad swearing in a new defence minister, the first images of the president since the bomb attack.
The location of the swearing-in ceremony of armed forces chief General Fahd Jassim al-Furayj was not clear, the BBC said.
There had been speculation that the president had also been killed or injured in the blast.
The president's brother-in-law, defence minister and head of his crisis team died in the attack, at a meeting at the national security headquarters in Damascus on Wednesday.
They were Defence Minister and ex-chief of staff General Daoud Rajiha; Deputy Defence Minister Assef Shawkat, who was married to Mr Assad's sister Bushra and the assistant to the vice-president and head of the crisis management office, General Hassan Turkomani.
Two other senior officials - Interior Minister Mohammad Ibrahim al-Shaar and National Security Bureau chief Hisham Ikhtiar - were wounded. It has since been reported Mr Ikhtiar has died, although that has not been confirmed by the Syrian regime.