The accusations "continue to be of grave concern," the Organisation for the Prohibition of Chemical Weapons (OPCW) said.

A fact-finding mission mandated "to establish the facts surrounding allegations of the use of toxic chemicals.... is investigating all credible allegations," the body said in a statement.

The latest fighting in Syria's seven-year-old civil war has seen an uptick in the alleged use of chemical weapons by the regime, including on the rebel-held enclave of Eastern Ghouta.

The fresh violence has sparked wide concern and drawn threats of military action from the United States.

"Any use of chemical weapons is a violation of the Chemical Weapons Convention and the hard-won international norm prohibiting these weapons," said OPCW chief Ahmet Uzumcu.

"Those responsible for their use must be held accountable. These abhorrent weapons have no place in the world today."

France said Wednesday that "all indications" suggested the Syrian regime was using chlorine weapons against rebel forces.

And the United States on Monday accused Russia of delaying the adoption of a UN Security Council condemnation of reported chlorine gas attacks in Syria that have left many injured, including children, in recent days.

US Ambassador Nikki Haley told the council there was "obvious evidence from dozens of victims" to corroborate the chlorine attacks in rebel-held Eastern Ghouta.

The OPCW's fact-finding mission was set up in April 2014, and its reports were sent to a joint UN and OPCW panel, known as the Joint Investigative Mechanism (JIM).

The JIM sought to identify those behind the deadly attacks, but the renewal of its UN mandate has been blocked since November by Russia.

A previous JIM probe however found that Syrian forces were responsible for an April 2016 sarin attack on the rebel-held town of Khan Sheikhun that killed scores of people.

The panel found that Syrian President Bashar al-Assad's forces had used chlorine in two attacks on rebel-held villages in 2014 and 2015.

It also determined that Islamic State (IS) jihadists had used mustard gas in 2015 in the country.