Secretary of State Rex Tillerson charged Tuesday that Russia's support for Syrian President Bashar Assad shows it has abandoned the effort to rid the world of chemical weapons.

“There is simply no denying that Russia, by shielding its Syrian ally, has breached its commitments to the United States,” Tillerson told reporters.

He offered that rebuke in response to a chlorine-gas attack in a rebel-held region of Syria that reportedly took place Monday. Russian officials have suggested that terrorists rather than Assad bear responsibility for such attacks since April, when President Trump authorized an airstrike against the Assad regime in retaliation for the use of chemical weapons.

Tillerson preempted that defense Tuesday by noting that Russia was supposed to have overseen the destruction of all chemical weapons in the country in 2014.

“Whoever conducted the attacks, Russia ultimately bears responsibility for the victims in East Ghouta and countless other Syrians targeted with chemical weapons since Russia became involved in Syria,” Tillerson said. “It has betrayed the Chemical Weapons Convention and UN Security Council Resolution 2218, and on these occasions has twice vetoed UN Security Council resolutions to enforce the Joint Investigative Mechanism and continue its mandate [to investigate attacks in Syria].”

The attack took place on the eve of a diplomatic summit in Paris, where Tillerson and other western officials are meeting to discuss the implementation of international bans on chemical weapons. It’s also just days removed from a United Nations Security Council meeting in which Ambassador Nikki Haley denounced Russia for vetoing the continuance of an investigative panel formed to probe chemical weapons attacks in Syria.

“[T]his attack in Syria should weigh heavily on their conscience,” Haley said Tuesday. “The United States will never stop fighting for the innocent Syrian children, women, and men who have become victims of their own government and those who continue to prop it up.”

Assad is trying to recapture one of the last major rebel outposts, even as all major players in the conflict are proceeding to a new phase of regional rivalry following the destruction of the Islamic State as a land-holding power. Russia has called for American forces to withdraw, and said the U.S. lacks a legitimate reason to operate in the country without Assad’s invitation.

“The U.S. are trying to form alternative bodies of authority on the vast parts of the Syrian territory,” Russian Foreign Minister Sergey Lavrov said Friday. “That is a fact, and that does contradict their own obligations.”

Tillerson has defended a continued presence in the country as being necessary to help bring peace to Syria and preclude any possible resurgence of ISIS. On Tuesday, he argued that the latest gas attack undermines Russia’s role in the country.

“Russia’s failure to resolve the chemical weapons issue in Syria calls into question its relevance to the resolution to the overall crisis,” he said. “At a bare minimum, Russia must stop vetoing and at least abstain from future Security Council votes on this issue.”