President Trump on Sunday condemned a Saturday chemical attack in Syria and criticized Russian President Vladimir Putin and Iran for backing Syrian President Bashar al Assad, who Mr. Trump called an “animal.”

“Many dead, including women and children, in mindless CHEMICAL attack in Syria. Area of atrocity is in lockdown and encircled by Syrian Army, making it completely inaccessible to outside world,” the president tweeted.

“President Putin, Russia and Iran are responsible for backing Animal Assad. Big price…to pay. Open area immediately for medical help and verification. Another humanitarian disaster for no reason whatsoever. SICK!” Mr. Trump said.

Dozens of people were reportedly killed on Saturday in a chemical attack in Douma amid continued tensions between opposition activists and Syrian government forces. The Syrian government denied the allegations.

The attack in Douma comes almost exactly a year after a chemical attack in the northern Syrian town of Khan Sheikhoun killed dozens of people. That attack prompted the U.S. to launch several dozen Tomahawk cruise missiles at a Syrian air base.

On Sunday, Mr. Trump blamed President Obama for Mr. Assad’s continued hold on power in the region, saying Mr. Obama failed to enforce his own “red line” on the use of chemical weapons.

“If President Obama had crossed his stated Red Line In The Sand, the Syrian disaster would have ended long ago! Animal Assad would have been history!” Mr. Trump tweeted.

Sen. Lindsey Graham said Sunday that Mr. Trump is now confronted with a defining moment of his presidency, amid talk that he would like to de-escalate the U.S. military presence in the region.

“President Trump can reset the table here,” Mr. Graham said on ABC’s “This Week.” “If it becomes a tweet without meaning, then he’s hurt himself with North Korea. If he doesn’t follow through and live up to that tweet, he’s going to look weak in the eyes of Russia and Iran.”

“So this is [a] defining moment, Mr. President,” he said. “You need to follow through that tweet - show resolve that Obama never did to get this right.”

‘Convulsions and pinpoint pupils’ in Douma

Douma is the last rebel stronghold in the suburbs of Damascus known as eastern Ghouta.

A chemical attack in eastern Ghouta in 2013 that was widely blamed on government forces killed hundreds of people, prompting the U.S. to threaten military action before later backing down.

Syrian opposition activists and rescuers said Sunday that the poison gas attack in Douma killed at least 40 people and that families were found suffocated in their homes and shelters, with foam on their mouths.

While the reports could not be independently verified, the opposition-linked Syrian Civil Defense were able to document 42 fatalities but were impeded from searching further by strong odors that gave their rescuers difficulties breathing, said Siraj Mahmoud, a spokesman for the group, which is known as the White Helmets.

A joint statement by the Civil Defense and the Syrian American Medical Society, a relief organization, said more than 500 people, mostly women and children, were brought to medical centers with difficulty breathing, foaming at the mouth, and burning of the eyes. It said patients gave off a chlorine-like smell. Some had blue skin, a sign of oxygen deprivation.

It said the symptoms were consistent with chemical exposure. One patient, a woman, had convulsions and pinpoint pupils, suggesting exposure to a nerve agent.

The Syrian Observatory for Human Rights said at least 80 people were killed in Douma on Saturday, including around 40 who died from suffocation. But it said the suffocations were the result of shelters collapsing on people inside.

The attacked occurred amid a resumed offensive by Syrian government forces after the collapse on Friday afternoon of a 10-day truce with the Army of Islam rebel group.

But in a statement posted on the state-run news agency SANA, the Syrian government strongly denied the allegations. It said the claims were “fabrications” by the Army of Islam, calling it a “failed attempt” to impede government advances.

Russia, the Syrian government most powerful ally — also denied any involvement in the alleged gas attack.

On Sunday, Maj. Gen. Yuri Yevtushenko was quoted by Russian news agencies as saying Russia was prepared to “promptly send Russian specialists in radiation, chemical and biological protection to Douma after its liberation from fighters to gather data that will confirm the fabricated nature of these statements.”

Mr. Yevtushenko said “a number of Western countries” are trying to prevent the resumption of an operation to remove Army of Islam fighters from Douma and “to this end they are using the West’s pet theme of the use of chemical weapons by Syrian forces.”

Syria denies ever using chemical weapons during the seven-year civil war, and says it eliminated its chemical arsenal under a 2013 agreement brokered by the U.S. and Russia after the attack in eastern Ghouta.

• This article is based in part on wire service reports.

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