PARIS (Reuters) - U.S. Secretary of State Rex Tillerson said on Tuesday the Syrian government may still be using chemical weapons against its own people following a suspected chlorine attack in a rebel enclave, and he said Russia bore ultimate responsibility.

Rescue workers in the rebel-held enclave of Eastern Ghouta, near to Damascus, accused government forces of using chlorine gas during bombardment of the area on Monday. The UK-based Syrian Observatory for Human Rights said at least 13 people had suffered suffocation in the incident.

President Bashar al-Assad’s government denies using chemical weapons, which it agreed to destroy in 2013 under an agreement brokered by Russia and the United States.

“Only yesterday more than 20 civilians, mostly children, were victims of an apparent chlorine gas attack,” Tillerson said after attending a conference on chemical weapons in Paris.

“The recent attacks in Eastern Ghouta raise serious concerns that Bashar al-Assad may be continuing to use chemical weapons against his own people.”

Eastern Ghouta is the last major rebel-held zone near Damascus and where at least 390,000 civilians have been besieged for four years.

“Russia ultimately bears responsibility for the victims in Eastern Ghouta and countless other Syrians targeted with chemical weapons since Russia became involved in Syria,” said Tillerson.

RUSSIA “BREACHED ITS COMMITMENTS”

Russia is a close ally of Assad, providing direct military support in Syria against the various rebel groups trying to oust him and also diplomatic cover in the U.N. Security Council.

Tillerson was speaking during a meeting hosted by France on an initiative to target those responsible for chemical attacks, largely in Syria, after an international investigation into who is to blame for chemical weapons attacks in Syria ended in November after Russian opposition.

A joint inquiry of the United Nations and the Organization for the Prohibition of Chemical Weapons (OPCW) determined that the Syrian government used the nerve agent sarin in an April 4, 2017, attack and also used several times chlorine as a weapon.

“Russia’s failure to resolve the chemical weapons issue in Syria calls into question its relevance to the resolution of the overall crisis,” Tillerson said, adding there was mounting evidence since April 2014 that Syria still had chemical weapons.

“There is simply no denying that Russia, by shielding its Syrian ally, has breached its commitments to the United States as a framework guarantor,” he added.

At the Paris meeting 24 countries agreed to work more closely on targeting those behind chemical weapons attacks and imposing necessary sanctions on them.

As a first step, France on Tuesday imposed unilateral sanctions on 25 people and entities, including from China, Lebanon and France, and among them importers and distributors of metals, electronics and lighting systems. It said the companies were helping to supply Syria’s chemical weapons programme.

“The current situation cannot continue,” French Foreign Minister Jean-Yves Le Drian said. “The criminals who have used and designed these barbaric weapons must know that they will not go unpunished.”