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.

'Multiple' chlorine

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.

Canadians released

More than 340,000 people have been killed in Syria's conflict, which began with anti-government protests but morphed into an ugly civil war.

Thousands of children are among the victims, and the UN's children agency on Monday decried yet another bloody month for Syria's children.

UNICEF said at least 83 children were killed in conflicts across the Middle East in January, including 59 in Syria alone.

In Arbin on Monday, an AFP correspondent saw the lifeless bodies of young children laid out on the floor in the local hospital.

And in nearby Zamalka, another devastated Eastern Ghouta town, a man could be seen carrying the body of his child, wrapped in a blood-soaked white sheet, through the streets.

In apparent retaliation for Monday's bombardment, rockets and mortars rained down on government-controlled districts of Damascus, Syria's state news agency SANA reported.

One woman was killed and four people were wounded in mortar fire on the Bab Touma neighbourhood and the capital's Mariamite Cathedral, a Greek Orthodox church, a police source told the agency.

Another person was killed in rocket fire on the regime-held part of Harasta district.

Also on Monday, two Canadian citizens who were being held by jihadist factions in the country's northwest were released to Turkish authorities.

Jolly Bimbachi and Sean Moore had crossed into Syria in December 2017 from Lebanon, where Bimbachi was fighting a custody battle for her two sons, she told AFP.

They were picked up by Hayat Tahrir al-Sham (HTS), an alliance dominated by Al-Qaeda's former Syrian affiliate, and handed over to a civilian authority.

“They're going to take us into Turkey, and in Turkey we are going to meet someone from the Canadian embassy,” Bimbachi said.