Regime bombardment killed nearly 30 people in a rebel enclave near Damascus on Monday, as Syria´s seven-year conflict left civilians paying a heavy price.
Residents across several Syrian battlefronts have reported escalating bombardment and have accused Syrian troops of deploying toxic chemicals against rebel-held zones.
The United States on Monday said there was "obvious evidence" of multiple chlorine gas attacks in recent weeks, including in the opposition-held enclave of Eastern Ghouta near Damascus.
On Monday, dozens of air strikes and artillery fire battered Eastern Ghouta, according to the Britain-based Syrian Observatory for Human Rights.
"Twenty-nine civilians were killed and dozens were wounded," said the war monitor´s head, Rami Abdel Rahman.
The deadliest raids on Monday hit a market in the town of Beit Sawa, killing 10 civilians including two children.
Another nine civilians, two of them children and one a local rescue worker, were killed in Arbin.
Eastern Ghouta is included in a de-escalation deal agreed last year by rebel ally Turkey and government supporters Iran and Russia.
But violence has ramped up there in recent weeks, and this month alone, chlorine is suspected of having been used on two occasions in munitions launched by the regime on Eastern Ghouta.
A third accusation of toxic gas use came from Idlib, an opposition-controlled province in the country´s northwest that also falls in a de-escalation zone.
Nearly a dozen people were treated for breathing difficulties on Sunday after Syrian government raids on the town of Saraqeb, the Observatory said.
Mohammad Ghaleb Tannari, a doctor in a nearby town, said his hospital had treated 11 people.
"All the cases we received had symptoms consistent with inhaling the toxic chlorine gas, including exhaustion, difficulty breathing, and coughing," he told AFP.
The United States and Russia clashed at the UN Security Council on Monday over a push by Washington to condemn reported chlorine gas attacks in Syria.
The US proposed a draft statement condemning the use of chemicals as a weapon, but Syrian government ally Russia added amendments that made no mention of the attack, according to a draft seen by AFP.
In the end no statement was adopted.
US ambassador Nikki Haley slammed Russia for balking at a statement that she described as a "simple condemnation of Syrian children being suffocated by chlorine gas".
But Russian ambassador Vassily Nebenzia hit back. "It´s completely clear to us the goal is to basically accuse the Syrian government of chemical weapons use where no perpetrators have been identified," he said.
The UN has found that Syria´s government carried out chlorine gas attacks in 2014 and 2015, and also used sarin against a town in Idlib last year.
Syria´s government has vehemently denied ever deploying chemical weapons in the country´s seven-year war.