While the Syrian government's use of nerve agents in densely populated opposition areas has drawn international censure and prompted President Trump to order missile strikes against a Syrian military base in April, chlorine attacks have continued unabated throughout the six-year war, according to monitoring groups.
Speaking in Paris, Tillerson blamed Russia for the apparent use of chlorine gas Monday in the besieged Damascus suburb of eastern Ghouta.
"Whoever conducted the attacks, Russia ultimately bears responsibility," Tillerson said of Monday's attack in eastern Ghouta and other suspected gas attacks since the Russian military began backing government forces in Syria in 2015.
Rescue workers said pro-government forces fired nine shells at dawn carrying suspected chlorine gas on a densely populated residential area in the Damascus suburb. Medical staff in the enclave said they had treated four women, seven children and 10 men with breathing difficulties and other symptoms consistent with exposure to the chemical weapon. Photos from the area circulated by local activists showed at least two infants breathing with the help of respirators as anxious parents watched.
Hours later, pro-government media said that rebel forces responded by shelling the Old City of Damascus, killing nine civilians, including a 3-year old child.
Separately, doctors from the Syrian American Medical Society, a nonprofit supporting hospitals across opposition-held parts of Syria, reported another attack on the northern province of Idlib. "Four people were treated in our hospital in Idlib with symptoms indicating exposure to chlorine," said Mohamed Katoub, a Turkey-based spokesman for the organization.
Violence has intensified in Idlib as the Syrian government presses an offensive against al-Qaeda-linked rebels there.
"When you hear the bombs, you brace yourself and wait. It is hard to describe the horrors we have seen coming through our doors this week," said a doctor in Idlib city, speaking on the condition of anonymity out of concern for the security of relatives living in government-held areas. "There was a little girl whose brain had fallen onto her pink T-shirt. And now we have the chemical victims, again. No one wants to stop this."
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.
Russia is a key backer of Syrian President Bashar al-Assad's government, providing both financial and military support, and has used its U.N. Security Council veto to block efforts to subject most alleged chemical assaults to U.N.-backed investigations.
Russian Deputy Foreign Minister Sergei Ryabkov called the allegations that Russia was obstructing the investigations into chemical attacks "dirty and mendacious," according to Russian news agency Interfax.
"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. Russia proposed Tuesday that the Security Council create a new inquiry to establish blame for chemical attacks in Syria, a move condemned by the United States as an attempt to distract from previous U.N. findings that Assad's military had carried out the April sarin attack.
"When Russia doesn't like the facts, they try and distract the conversation. That's because the facts come back over and over again to the truth Russia wants to hide — that the Assad regime continues to use chemical weapons against its own people," said Nikki Haley, the U.S. ambassador to the United Nations.
Tillerson was speaking at a meeting of 29 countries trying to identify, shame and punish those who use chemical weapons. He 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 Nations has accused Assad's government of using the deadly nerve agent sarin in a daybreak assault on the northern town of Khan Sheikhoun in April. That attack killed at least 83 people and flooded surrounding hospitals with hundreds more casualties.
The Syrian American Medical Society has recorded 194 chemical attacks across Syria since 2012, most involving chlorine-like substances.
"The same dozen or so towns, hotbeds of opposition and militant activity, are struck over and over again," said Tobias Schneider, an independent security analyst tracking the use of chemical weapons in Syria. "The Assad regime persists in its use of chlorine simply because it is a cheap and expedient way of conducting population warfare. While the United States last year chose to finally enforce its red line against the much more lethal nerve agent sarin, no such ultimatum has been issued against choking agents."
Syria has denied using chemical weapons against the country's shrinking opposition-held enclaves. With substantial support from its Russian and Iranian allies, the Syrian government has isolated rebel forces in a handful of pockets in the north and south of the country.
Tillerson's comments Tuesday reflect the growing difficulty faced by the United States in influencing the course of Syria's war.
"Moscow has dug down in a position that basically means they deny everything, even vetoing U.N. investigations that contradict that line. There's not a lot of room for productive engagement left, and the Americans seem to have lost all faith in Moscow's intention to stand by its word on any deals reached," said Aron Lund, a fellow at the New York-based Century Foundation.
The Syrian government was supposed to have surrendered its stocks to international inspectors in 2013, following a sarin attack in eastern Ghouta that is thought to have killed more than one thousand people. Investigators and Western diplomats have long suspected that stockpiles were secretly withheld or that new batches were produced.
Under the terms of a deal brokered by the United States and Russia, the Organization for the Prohibition of Chemical Weapons, a global watchdog, has conducted routine inspections of Syrian military research sites since 2013. But those missions have often been hampered by Syrian government restrictions on where and when the teams can travel.
"This issue doesn't get a lot of attention compared to the chemical attacks themselves, but it seems to me that people involved with these issues view them as even more important," said Lund. "To most countries in the neighborhood, it's more important to make sure Syria doesn't go back to producing nerve gas in bulk again. Many are convinced that Assad kept a small stockpile, but nothing like what Syria had before 2013."
The United States has urged Russia to force Assad to join U.N.-sponsored peace negotiations 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 calendar year.
Tillerson dismissed Moscow's authority to be further involved in attempts to bring peace to Syria, saying "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 or at least abstain from future Security Council votes on the use of chemical weapons in Syria.
Shortly before Tillerson spoke, Haley also criticized Russia regarding the attacks. She singled out Russia's November veto of a resolution to renew the Joint Investigative Mechanism (JIM), a technical group charged with investigating 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."
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 and its meddling in the 2016 U.S. election and now 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.
Morello reported from Washington.