Russia has said bomb attacks which killed at least 140 people in Syria were aimed at “subverting attempts” to reach a political settlement.
Russia’s foreign ministry condemned the “atrocious crimes of extremists”.
Sunday’s attacks hit the Shia shrine of Sayyida Zeinab, south of Syria’s capital Damascus, and the city of Homs.
So-called Islamic State (IS) said it carried out the attacks. Both targeted areas dominated by Islamic minorities reviled by IS.
Four blasts in Sayyida Zeinab killed at least 83 people, according to state media. A monitoring group reported that 57 people, mainly civilians, were killed in a double car bombing in Homs.
The UK-based activist group Syrian Observatory for Human Rights (SOHR) put the toll from the Damascus attacks at 120 and said they were among the deadliest to occur during the whole of Syria’s civil war.
SOHR also reported on Monday that heavy fighting had cut off the government’s only supply route to the northern city of Aleppo.
IS and other Islamist militia had cut the road between Aleppo and the town of Khanasser to the south-east, it said.
More than 250,000 Syrians have been killed in the conflict.
Some 11 million others have been forced from their homes, of whom four million have fled abroad – including growing numbers who are making the dangerous journey to Europe.
The Sayyida Zeinab district is the location of Syria’s holiest Shia Muslim shrine, said to contain the grave of the Prophet Muhammad’s granddaughter.
The district was hit by suicide attacks last month that left 71 people dead and which IS fighters also said they had carried out.
On Sunday US Secretary of State John Kerry said a “provisional agreement” had been reached with Russia on a partial truce.
However he admitted issues remained to be resolved and said he did not expect any immediate change on the ground.
Earlier this month, world powers involved in the crisis in Syria agreed to seek a “cessation of hostilities”, but the Friday deadline came and went.
In Homs, the blasts happened in a predominantly Alawite district, the sect to which President Bashar al-Assad belongs.
One of the early centres of the uprising against President Assad, Homs was once dubbed the “capital of the revolution”.
But rebels left the city late last year under a ceasefire deal, leaving the city in government hands.
The latest blasts came as President Bashar al-Assad told reporters Syrian refugees should not be scared of returning home.
Mr Assad, who has long been accused of persecuting his own people, said ordinary Syrians who had fled the conflict due to the “standard of living that has been deteriorating drastically” could go back without fear of action by the government.