Twenty-five civilians are reported to have been killed in air strikes on two towns in the rebel-held Eastern Ghouta area outside Syria’s capital, Damascus.
The Syria Civil Defence said one of its rescue workers was among 18 people who died when what witnesses identified as Russian jets bombed Misraba.
Another six people died in suspected Russian strikes on Arbin, it added.
Some 400,000 people in the area have been under siege by Russian-backed Syrian government forces since 2013.
Last week, the government permitted Red Cross teams to evacuate 29 critically-ill patients from the Eastern Ghouta as part of a deal that saw rebels release the same number of prisoners.
Hundreds more patients are in urgent need of evacuation from the enclave, where there are only 107 doctors and severe shortages of medical supplies.
The Syria Civil Defence said Russian jets fired “seismic missiles” on residential areas of Misraba overnight, causing several buildings to collapse.
Nine women and two children were among the 18 people killed, it said, adding that the number might rise because some of the injured were in a critical condition.
One of the organisation’s volunteer rescue workers – who are commonly known as the White Helmets – died after a “double-tap” strike on one building.
The Syria Civil Defence said two children and two women were among the six people killed in the air strikes on Arbin, and that another three civilians were killed by government shellfire in Beit Sawa.
The Syrian Observatory for Human Rights, a UK-based monitoring group, put the death toll from the three towns at 23, and also said the warplanes that attacked Misraba and Arbin were believed to have been Russian.
There was no immediate comment from the Russian military, which is providing air support to government forces in the area but insists it has never targeted civilians.
The Eastern Ghouta is designated a “de-escalation zone” by Russia and Iran, the government’s other main ally, along with Turkey, which backs the rebels.
But hostilities intensified in mid-November, when the Syrian military stepped up air and artillery attacks on the enclave in response to a rebel offensive.
Earlier this week, rebels overran a major military base in the heart of the Harasta district, triggering heavy air strikes on surrounding rebel-held territory.
In a separate development, the Russian defence ministry denied a report in the newspaper Kommersant that seven Russian military planes had been destroyed by rebel shelling on Sunday of the Hmeimim airbase, near Latakia.
Kommersant cited two sources as saying that four Su-24 bombers, two Su-35s fighters, an An-72 transport plane were hit in the attack by “radical Islamists”, and 10 Russian military personnel injured.
The Tass news agency cited the defence ministry as saying that two personnel were killed by mortar fire but that no military hardware was lost.