Syrian activists say at least 80 people have died in government air strikes on a marketplace in the rebel-held town of Douma, near Damascus.
Around 200 people were reportedly injured in the attack.
Government forces have been regularly attacking Douma and its surrounding areas in recent months with air strikes and helicopter barrel bombs.
Hundreds of civilians have been killed alongside opposition fighters.
Images of the aftermath of Sunday's strikes show dead bodies lined up on the floor, including some of children.
Unverified footage uploaded by activists showed a marketplace destroyed, with surrounding buildings in ruins and vehicles on fire.
The Local Co-ordination Committees, a network of anti-government activists, said that rescue workers were digging through the rubble in search of survivors.
A Douma-based activist told the Associated Press news agency that the situation was "catastrophic", adding that clinics in the area were full and many of the wounded were being rushed in civilian cars to other medical facilities since ambulances were overwhelmed.
The government strikes are aimed at stopping rebel groups, including the Islamist rebel group Jaysh al-Islam, from firing rockets from Douma into Damascus, which is only about 7 miles (11 km) away.
An air strike on Wednesday reported killed at least 37 civilians in Douma, including four children.
The latest reported strikes coincide with the first visit to the country by the UN humanitarian chief Stephen O'Brien since he took up the post in May.
United Nations figures show that over 220,000 people have been killed and more than nine million displaced in Syrian civil war since it began four years ago.
The conflict began with a wave of peaceful protests against President Bashar al-Assad in March 2011, but by the end of that year had descended into armed confrontation between the government and armed opposition groups.
The war has seen the emergence of numerous heavily armed jihadist groups within Syria, including Jaysh al-Islam, Islamic State (IS) and al-Qaeda offshoot al-Nusra Front.
Recently some of the anti-Assad forces have begun to fight each other, with IS in particular being opposed by other Islamist factions and by fighters from the Kurdish minority along the country's borders with Turkey and Iraq.