The Syrian Observatory for Human Rights said on Monday that the death toll from Sunday's DAESH suicide attack has risen to more than 70.
A car bombing and twin suicide blasts hit the main Shiite neighbourhood of the Syrian capital, Damascus, while UN-mediated peace talks continue in Geneva.
The peace talks between representatives of the Syrian regime and the opposition is the first in two years.
State television reported that the blasts took place in the Sayeda Zeinab District, where Syria’s most important Shiite shrine is located. The footage showed burning buildings and car wreckage in the neighbourhood.
The Syrian state news agency, SANA, has put the death toll from the attack at more than 50.
The British-based Observatory, which monitors the war using contacts on the ground, said that the attack had targeted a military bus carrying Shiite militiamen who were changing guard, and that 42 of the dead were fighters allied to the government.
The Lebanese Shiite militant group, Hezbollah, and other Iraqi and Iranian militias have a strong presence in Sayeda Zeinab, which is a site of pilgrimage for Shiites from Iran, Lebanon and other parts of the Muslim world.