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Syrian opposition activists and aid groups said Sunday that dozens of people had died in a suspected chemical weapons attack on the city of Douma.

President Donald Trump described the attacks as “mindless” in a series of tweets on Sunday morning and, in an unusual criticism of the Russian president, said that “President Putin, Russia and Iran are responsible for backing Animal Assad. Big price to pay.” Though Trump had last week expressed eagerness to get out of Syria, a White House official said Sunday a military response remains on the table.

The alleged attack occurred late on Saturday amid a resumed offensive by Syrian government forces after the collapse of a truce with the Army of Islam rebel group. Douma is the last rebel stronghold in eastern Ghouta, a region that has been subjected to an intensive bombing campaign by Syrian President Bashar al-Assad's forces, which are backed by Russia.

The 10-day truce between the two sides collapsed over a disagreement regarding the evacuation of the Army of Islam fighters from the city. Violence resumed days after hundreds of opposition fighters and their relatives left Douma toward rebel-held areas in northern Syria.

Both the Syrian and Russian governments denied any involvement in the alleged chemical attack on Sunday.

The State Department says it is monitoring reports of the possible chemical weapons attack.

Local activists told NBC News that they saw dozens of dead bodies, many of whom were children, in the underground shelters across the city. One activist said he saw at least 40 bodies in a basement and said he smelled very strong gas, adding that it was "definitely a chemical attack."

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Another activist said he saw at least 25 bodies, most of whom he said were children. “Whole families, mothers and little children and babies, they were all dead," he said. “They tried to escape death, but here in Douma, there is death is everywhere."

The White Helmets, a Syrian civil defense organization, tweeted several photos and videos of alleged victims Saturday night.

A joint statement released Sunday by the Syrian American Medical Society said more than 42 casualties were found dead in their homes. It added that "more than 500 cases — the majority of whom are women and children — were brought to local medical centers with symptoms indicative of exposure to a chemical agent."

The claim has not been verified by NBC News.

If confirmed, the use of chemical weapons would be another instance of the banned substances alleged to have been used in Syria’s brutal civil war.

A spokesman for the U.N. secretary-general said: "While the United Nations is not in a position to verify these reports, the secretary-general notes that any use of chemical weapons, if confirmed, is abhorrent, and requires a thorough investigation."

Nine U.N.-member countries — including the United States — planned to hold an emergency Security Council meeting Monday to "demand immediate access for first responders, support an independent investigation into what happened, and hold accountable those responsible for this atrocious act,” the U.S. ambassador to the United Nations, Nikki Haley, said in a statement.

The possible chemical attack comes a year after Trump launched an assault on a Syrian air base after what U.S. officials said was a chemical attack on the town of Khan Sheikhoun. Trump said at the time that the attack was meant to deter further Syrian use of illegal weapons.

“This is one of those issues on which every nation, all peoples, have all agreed, and have agreed since WWII, is an unacceptable practice. … I wouldn’t take [military action] off the table," White House Homeland Security adviser Tom Bossert said on ABC's "This Week" Sunday.

UK, France, US, Poland, Netherlands, Sweden, Kuwait, Peru and Cote d’Ivore have called an emergency meeting of #UNSC to discuss reports of chemical weapons attack in #Syria. Meeting expected on Monday. — UK at the UN 🇬🇧 (@UKUN_NewYork) April 8, 2018

State Department spokeswoman Heather Nauert said Saturday that Russia, whose involvement in the conflict has helped turn the tide in favor of the Assad regime, "ultimately bears responsibility for these brutal attacks."

"The United States calls on Russia to end this unmitigated support immediately and work with the international community to prevent further, barbaric chemical weapons attacks," she said in the statement.

Pope Francis also referred to reports of the deaths in Syria during his traditional Sunday blessing. “There is not a good or a bad war, and nothing can justify such instruments that exterminate defenseless people and populations,” he said.

The Syrian government disputed any reports of gas attacks, saying the allegations are an attempt by rebels to stop the army from advancing, according to Syrian state news agency SANA. Russia also denied the reports in a statement from its Foreign Ministry on Sunday morning.

"Fake news on the use of chlorine or other chemical agents by the government forces continue," the ministry said in a statement provided to the TASS news agency.

"The goal of these false speculations, which are not substantiated by any facts, is to cover up terrorists and irreconcilable radical opposition, which opposes political settlement, and to simultaneously try to justify potential external military strikes," the statement added.

After Trump said last week that he wanted to withdraw U.S. troops from Syria, a senior administration official told NBC News that in a meeting with his national security team on Tuesday the president reluctantly agreed to keep forces on the ground for an undetermined period of time to defeat Islamic State.

The United States has around 2,000 troops in Syria. The official said Trump pressured his team to end U.S. involvement in the conflict as soon as possible.

The complex, multifront war has involved supporters of the regime, like Russia, Iran and Tehran-backed Lebanese militia Hezbollah, and regional powers seeking to unseat Assad, such as Saudi Arabia, the United Arab Emirates and Turkey.