An Assad regime bombardment killed 16 civilians on Saturday across the besieged opposition enclave of Eastern Ghouta, a monitor group said.
Eastern Ghouta has been under regime siege since 2013 and its estimated 400,000 inhabitants are suffering severe shortages of food and medicine.
On Saturday, a barrage of Syrian artillery fire hit a market in the main Eastern Ghouta town of Douma, according to the Syrian Observatory for Human Rights.
"Nine people were killed, including a woman," the British-based war monitor said.
Another six civilians were killed in artillery fire on the towns of Hammuriyeh, Arbeen and al-Marj. Regime air strikes killed a child in the town of Zamalka.
The United Nations has said some 500 people are in critical condition inside Eastern Ghouta and need to be evacuated for urgent medical treatment.
Twenty-nine patients, mainly children, were allowed out in December to get treatment under a deal struck between the regime and opposition, but several have already returned.
Eastern Ghouta was one of four "de-escalation zones" agreed under a deal agreed last year between Turkey, Iran and Russia.
But the opposition stronghold remains the target of intense air strikes and a crippling government siege.
Syria's conflict erupted in March 2011 with protests against the Assad regime but has since evolved into a ferocious civil war.