United Nations (UN) war crimes investigators on Tuesday said they were investigating fresh reports that chemical weapons were being used in rebel-held zones in war-ravaged Syria.

The UN Commission of Inquiry on the human rights situation in Syria voiced alarm that it had received “multiple reports — which it is now investigating — that bombs allegedly containing weaponised chlorine have been used in the town of Saraqeb in Idlib and in Douma in Eastern Ghouta".

Residents in Eastern Ghouta, near Damascus, and in the northwestern Idlib province have accused Syrian troops of using the toxic weapons in recent weeks.

The United States (US) 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.

Eastern Ghouta, where an estimated 400,000 people are living under a crippling government siege, 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.

The UN commission, headed by Brazilian diplomat Paulo Pinheiro, was last September the first UN body to officially blame Damascus for a chemical attack that killed more than 80 people in Khan Sheikhun in Idlib five months earlier.

The UN has also determined that Syria's government carried out chlorine gas attacks in 2014 and 2015, a charge Damascus has vehemently denied.

More than 340,000 people have been killed in Syria's conflict, which began in 2011 with anti-government protests but morphed into a brutal civil war, pulling in world powers and attracting jihadist fighters from around the globe.