Shelling by the Syrian regime on the rebel-held area of Eastern Ghouta near Damascus Friday killed at least 19 civilians, among them six children, a monitor said.
The deaths came amid an escalating cycle of tit-for-tat attacks between regime forces and rebels holding the enclave on the Syrian capital's eastern outskirts.
Rebel shelling on Friday killed three civilians.
According to the Syrian Observatory for Human Rights, 52 civilians have been killed since Tuesday, most of them in Eastern Ghouta, which has been besieged since 2013 and where humanitarian conditions are dire.
Thirteen people, including five children and three emergency workers, were killed in regime shelling and air strikes in Douma, the Eastern Ghouta area's main town, Observatory chief Rami Abdel Rahman told AFP.
Elsewhere in Eastern Ghouta, another six people were killed in regime air strikes, the monitor said.
On Tuesday, the Ahrar al-Sham group, which has positions in Harasta, attacked a regime military base in the area, which is supposed to be a "de-escalation zone" under a deal between Russia, Iran and Turkey to ease the level of violence.
The fighting on that front has left at least 37 dead on the regime side, according to the Observatory, a toll the regime has not confirmed. Abdel Rahman said "dozens" of Islamist rebels were also killed.
In a hospital in Douma, doctors and nurses were treating a continuous flow of the wounded as the sounds of crying children echoed through the facility, an AFP correspondent said.
An elderly man with greying hair sought to calm a little girl in tears, her clothes covered in blood, while the bodies of three children killed in the strikes lay inert on a metal table.
Two other injured children sat on a bench, silent, their eyes wide, apparently still in shock. One had a bandaged foot.
Another wounded person had a bandage wrapped around his head, but blood had soaked through it.
On a white hospital bed, Abu Hisham's face contorted in pain as he called out for his wife and children, who had been killed.
"Iman, where are the children?" he cried.
In retaliation for the latest deadly Ghouta shelling, rebels fired rockets into Damascus on Friday, killing three civilians, the same source said.
Six were killed the previous day, including Syria's national karate coach Fadel Fadi, who died of his wounds after being hit by shrapnel as he left his Damascus sports club, the state-run SANA news agency reported.
More than 330,000 people are estimated to have been killed in the Syrian war, which began in 2011 as the regime brutally crushed anti-government protests.
Millions have been displaced.