Secretary of State Rex Tillerson said Tuesday that Russia is responsible for recent gas attacks against civilians in Syria and called on Moscow to refrain from voting on U.N. Security Council measures about chemical weapons in the country.

Speaking in Paris, Tillerson blamed Russia for the apparent use of chlorine gas Monday in a rebel stronghold in suburban Damascus. Women and children were among 20 victims who reportedly suffered gas inhalation and were treated for breathing problems. Syria has denied using poisonous gas in its push against the rebels.

"Whoever conducted the attacks, Russia ultimately bears responsibility," Tillerson said of Monday's assault in East Ghouta and other suspected chemical attacks since Russia began backing government forces in Syria.

Tillerson said Russia's failure to rid Syria of chemical weapons violates a 2013 disarmament agreement it made with the United States.

"There is simply no denying that Russia, by shielding its Syrian ally, has breached its commitments to the United States," he said.

Tillerson was speaking at a meeting of 29 countries trying to identify, shame and sanction those who use chemical weapons. Tillerson said their initiative puts perpetrators "on notice."

"You will face a day of reckoning for your crimes against humanity, and your victims will see justice done," he said. "The choice is yours. The people of East Ghouta are watching, and the rest of the world is watching as well."

The United States has urged Russia to force Syrian President Bashar al-Assad to join peace negotiations sponsored by the United Nations that are scheduled to resume later this week in Vienna. Moscow has worked with Iran and Turkey to conduct parallel peace talks in Astana, Kazakhstan, and the Russian city of Sochi as the civil war stretches into its seventh year.

Tillerson dismissed Moscow's authority to be further involved in attempts to bring peace to Syria, saying that "Russia's failure to resolve the chemical weapons issue in Syria calls into question its relevance to the resolution of the overall crisis."

He said Russia must stop vetoing future Security Council votes on the use of chemical weapons in Syria, or must at least abstain.

Shortly before Tillerson spoke, Nikki Haley, the U.S. ambassador to the United Nations, also criticized Russia in connection with the attacks. She singled out Russia's November veto of a resolution to renew the Joint Investigative Mechanism (JIM), a technical group charged with probing the use of chemical weapons in Syria.

"When Russia killed the JIM, they sent a dangerous message to the world — one that not only said chemical weapons use is acceptable but also that those who use chemical weapons don't need to be identified or held accountable," she said in a statement. "If these reports are true, this attack in Syria should weigh heavily on their conscience."

Russian Deputy Foreign Minister Sergei Ryabkov called the allegations that Russia was obstructing the investigation into chemical attacks "dirty and mendacious," according to Russia's Interfax news agency.

"This is a blatant and by any standard outrageous example of the American side's manipulating facts and ignoring what we've been saying for several years," Ryabkov told Interfax.

Tillerson's remarks underscored his growing role as the chief critic of Russia within the administration. Though Russian President Vladi­mir Putin awarded Tillerson an Order of Friendship medal for his work on an oil deal when he headed ExxonMobil, Tillerson has frequently berated Moscow for its support of separatists in Ukraine, its meddling in the 2016 U.S. election, and now, its role in Syria.

Last week, Tillerson said the United States would stay engaged in Syria, militarily and diplomatically, for an indefinite period to counter the threat posed by Iran.

The United States also has expressed growing concern about a Turkish assault against Kurdish militias in Syria. The United States has backed Syrian Kurds in the fight against the Islamic State, but Ankara believes they are allied with Turkish Kurds who are considered terrorists.

Anton Troianovski in Moscow contributed to this report.