French President Emmanuel Macron on Friday pressed Russia's Vladimir Putin to do all he can to ensure the Syrian government ends a deteriorating humanitarian crisis in eastern Ghouta and Idlib, the Elysee Palace said in a statement.

In a telephone call, Macron told Putin it was imperative that peace talks make progress and expressed concern over signs that chlorine bombs had been used against civilians in recent weeks.

"The president stressed the need to overcome obstacles blocking negotiations and start a credible political process in the coming weeks under the auspice of the United Nations toward rebuilding peace, stability and unity in Syria," the French presidency statement said.

Chemical weapons a 'red line'

Human rights group says 229 people killed in four days 1:24 Standing beside Putin in Versailles last summer, Macron said any use of chemical weapons represented a "red line" and that Paris could launch unilateral airstrikes against targets in Syria if it were crossed.

Meanwhile, earlier this week, the UN called for an immediate humanitarian ceasefire in Syria of at least a month, as heavy airstrikes were reported to have killed scores of people in the last major rebel stronghold near Damascus.

UN war crimes experts also said they were investigating several reports of bombs allegedly containing chlorine gas being used against civilians in the rebel-held towns of Saraqeb in the northwestern province of Idlib and Douma in the Eastern Ghouta suburbs of Damascus.

The Syrian government denies using chemical weapons.

The Syrian Observatory for Human Rights said Tuesday's bombardment of Eastern Ghouta had killed 63 people. A local official, Khalil Aybour, put the toll at 53. On Monday, airstrikes killed 30 people in Eastern Ghouta, the observatory said.

"Today there is no safe area at all. This is a key point people should know: there is no safe space," Siraj Mahmoud, head of the Civil Defence rescue service in opposition-held rural Damascus, told Reuters.