Fresh regime strikes on a besieged rebel-held enclave near Damascus killed 35 civilians yesterday despite mounting Western pressure on Syrian President Bashar al-Assad.

The bloodshed came a day after another 31 civilians were killed in Eastern Ghouta and as the United Nations pleaded for one-month truce in the seven-year-old conflict to allow for aid deliveries.

Fighting also raged in the northwestern province of Idlib, where the UN said the violence "made a mockery" of the de-escalation zones agreed last year in a bid to pave the way for an end to the war.

The latest casualties in Ghouta, on paper also a de-escalation zone, came as Washington threatened military action over the reported use of chemical weapons in the enclave, which regime and allied forces have besieged since 2013.

The death toll rose from an initial report of 16 "because the strikes are continuing -- some new victims died in Douma and some critically wounded people died of their wounds," the Syrian Observatory for Human Rights said.

Observatory chief Rami Abdel Rahman added that few rebels were among the dead because they rarely left their tunnels and had better protection from air strikes than civilians, estimated to number around 400,000 in Ghouta.

In apparent retaliation, rockets were fired on Damascus's Bab al-Touma neighbourhood, killing three civilians, the state news agency SANA reported.

Although less deadly, regime attacks involving suspected chlorine-filled munitions on Ghouta have also been on the up in recent weeks.

The US State Department said on Monday it had recorded six suspected chemical attacks in Syria in the past 30 days, reported AFP.

Washington, which last year launched cruise missiles on a regime airfield in retaliation for a nerve agent attack in Khan Sheikhun that killed scores of people, has threatened more military action.

Meanwhile, Turkish President Tayyip Erdogan ramped up his verbal assault on the US role in Syria yesterday, saying US forces should leave Manbij, a Syrian city held by YPG-allied forces with support from the US-led coalition.

He said the United States was working against the interests of Turkey, Iran and maybe Russia in northern Syria, reported Reuters.

The UN said it was looking into the reports of chemical attacks and condemned what it described as an escalation in violence in Ghouta and Idlib.

"Over the last 48 hours, the scale and ferocity of attacks has increased dramatically," the UN-mandated Independent International Commission of Inquiry said in a statement.

In Idlib, the last province in the country to largely escape government control, daily fighting is also claiming an growing human toll and displacing thousands.

The UN commission said at least three hospitals were hit in recent strikes and shelling.