At least 25 civilians were killed on Sunday in Syrian regime airstrikes on Eastern Ghouta near the capital Damascus, said the Syrian Observatory for Human Rights.
Dozens of people were injured in the attacks, it added.
The deadliest raids hit the town of Hammuriyeh, killing 17 civilians including six children, the Britain-based monitor reported.
Other air strikes on the towns of Arbeen, Beit Sawa, Harasta and Misraba killed eight civilians.
The Observatory said at least 75 people were wounded and the death toll was likely to rise given the critical wounds suffered by dozens of people.
In Hammuriyeh, an AFP photographer saw men carrying the body of a victim wrapped in a blue blanket across a rubble-littered street, a shoeless foot sticking out.
The strikes appeared to hit a modest residential neighborhood, smashing a small convenience store and caking a vegetable stand in a layer of dust.
Eastern Ghouta, one of the last remaining rebel strongholds in Syria, has been under a heavy regime siege since 2013 that has caused drastic food and medical shortages.
An estimated 400,000 people still live in the enclave, and the United Nations has warned that hundreds are at risk of death if they are not evacuated for urgent medical treatment.
The area is one of four "de-escalation zones" agreed earlier this year by regime allies Russia and Iran and rebel backer Turkey aimed at curbing violence across war-ravaged Syria.
Syria's regime agreed last week to a ceasefire in the region to coincide with the resumption of UN-led peace talks in Geneva.
But heavy bombardment of the opposition enclave near the capital has resumed, with another 11 civilians killed on Saturday.
The Observatory said Sunday's deaths took the number of civilians killed in regime bombardment of Eastern Ghouta to at least 191, including 43 children, in nearly three weeks.