The U.S. State Department said on Saturday reports of mass casualties from an alleged chemical weapons attack in Douma, Syria, were "horrifying" and would demand an international response if confirmed.
"These reports, if confirmed, are horrifying and demand an immediate response by the international community," State Department spokeswoman Heather Nauert said in a statement.
Citing a history of chemical weapons use by the government of Syrian President Bashar Assad, Nauert said Assad's government and its backer Russia needed to be held accountable and "any further attacks prevented immediately."
"Russia, with its unwavering support for the regime ultimately bears responsibility for these brutal attacks," Nauert said.
Russia dismissed reports of the deadly chemical attack in Douma, Interfax news service reported on Sunday, citing Russia's Ministry of Defense.
"We decidedly refute this information," Major-General Yuri Yevtushenko, head of the Russian peace and reconciliation center in Syria, was cited as saying.
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"We hereby announce that we are ready to send Russian specialists in radiation, chemical and biological defense to collect information, as soon as Douma is freed from militants. This will confirm the trumped-up nature of these statements," Yevtushenko is quoted as saying.
Damascus has denied mounting any such attack and has blamed rebels for spreading false news.
The chemical attack on a rebel-held town in Syria's eastern Ghouta killed dozens of people, a medical relief organization and rescue workers reported.
Medical relief organization Syrian American Medical Society said 41 people had been killed, with other reports putting the death toll much higher. The Syrian Observatory for Human Rights said at least 80 people were killed, including around 40 who died from suffocation. The civil defense rescue service, also known as the White Helmets, put the death toll as high as 150 on one of its Twitter feeds.
The Russian-backed Syrian state denied government forces had launched any chemical attack as the reports began circulating on Saturday night and said rebels in the eastern Ghouta town of Douma were in a state of collapse and spreading false news.
Later, State TV reported that the Syrian government was ready to start negotiating with Jaish Al-Islam in Douma, quoting an official source.
Last year, a joint inquiry by the UN and the Organization for the Prohibition of Chemical Weapons (OPCW) found the Syrian government was responsible for an April 4, 2017 attack using the banned nerve agent sarin in the opposition-held town of Khan Sheikhoun, killing dozens of people.
The inquiry had previously found that Syrian government forces were responsible for three chlorine gas attacks in 2014 and 2015 and that Islamic State militants used mustard gas.