Damascus — Fresh air strikes by the Syrian military on the rebel-held enclave of Eastern Ghouta, near Damascus, killed at least 16 civilians on Tuesday, a monitoring group said.

"There were 16 killed, two of them children, in intensive air raids carried out by the regime against several regions of Eastern Ghouta," Rami Abdel Rahman, the head of the Syrian Observatory for Human Rights, said.

The Britain-based war monitor said Syrian government aircraft and shelling hit the towns of Harasta, Hamouriya, Douma and other parts of the eastern Ghouta pocket near the capital.

The United Nations in Syria appealed on Tuesday for a one-month truce in the conflict to allow aid to reach hundreds of thousands of civilians in urgent need of assistance.

In a statement issued in Damascus, the UN said a humanitarian crisis was unfolding in several parts of the country to which it was not being given access.

The UN's humanitarian coordinator and top envoys in Syria called "for an immediate cessation of hostilities lasting for at least one month throughout Syria."

The measure would "enable the delivery of humanitarian aid and services, evacuation of the critically sick and wounded, and alleviation of people's suffering, to the extent possible, wherever they are," the UN said.

On Monday, regime bombardment killed nearly 30 people in a rebel enclave near, 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.

On Jan. 20, Turkish forces backing Syrian rebels launched a major operation against the Kurdish-controlled region of Afrin in the north of the country.

Large numbers of people who were displaced by last year's operations against the Islamic State group in the Raqa and Deir Ezzor regions are still in urgent need of humanitarian relief.

The UN said existing agreements on the delivery of aid were not being honored and stressed that "if access was granted, three convoys could be dispatched each week, reaching over 700,000 people in these areas in two months."

While the fighting is abating in some parts of the country, the humanitarian crisis is still in full swing and aid groups predict a further 1.5 million people will be displaced in 2018. — Agencies