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BEIRUT -- Suspected poison gas was used to attack the last remaining foothold for the Syrian opposition in the eastern suburbs of Damascus, killing at least 40 people, including families found in their homes and shelters, opposition activists and local rescuers said Sunday.

The attack on the besieged town of Douma came 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.

President Trump blamed Syrian government forces for what he called a "mindless CHEMICAL attack" and warned there would be a "big price to pay." He did not elaborate. In a series of tweets, Mr. Trump held Russia and Iran, Syrian President Bashar Assad's chief sponsors, responsible.

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The Syrian government denied the allegations, calling them fabrications.

First responders entering apartments in Douma late Saturday said they found bodies collapsed on floors, some foaming at the mouth. The opposition's Syrian Civil Defense rescue organization said the victims appeared to have suffocated.

This image released early Sun., April 8, 2018, by the Syrian Civil Defense White Helmets, shows victims of an alleged chemical weapons attack collapsed on the floor of a building in the rebel-held town of Douma, near Damascus, Syria. Syrian Civil Defense White Helmets / AP

They did not identify the substance used, but the civil defense organization, also known as the White Helmets, and the Syrian American Medical Society, a medical relief organization, said survivors treated at clinics smelled strongly of chlorine.

Those reports could not be independently verified because of a government blockade around the town.

Hours after the attack, the Army of Islam rebel group agreed to surrender the town and evacuate their fighters to rebel-held northern Syria, Syrian state media reported. The group also agreed to give up its prisoners, a key demand of the government.

The government agreed to halt its assault after three days of indiscriminate air and ground attacks.

"There's nothing left for civilians and fighters. We don't have anything to stand fast," said Haitham Bakkar, an opposition activist inside the town. He spoke to The Associated Press via WhatsApp.

"People now are going out in the streets looking for their loved ones in the rubble," Bakkar said. "And we don't have any space left to bury them."

This image made from video released by the Syrian Civil Defense White Helmets, which has been authenticated based on its contents and other AP reporting, shows a medical worker giving toddlers oxygen through respirators following an alleged poison gas attack in the opposition-held town of Douma, in eastern Ghouta, near Damascus, Syria, on Sun., April 8, 2018. Syrian Civil Defense White Helmets / AP

U.S. State Department spokeswoman Heather Nauert said that Washington was closely following "disturbing reports" of the alleged chemical weapons attack in Douma.

"These reports, if confirmed, are horrifying and demand an immediate response by the international community," she said in a statement late Saturday. Last week, U.N. Ambassador Nikki Haley issued renewed warnings to Assad against the use of chemical weapons. A spokesman for the U.K. mission to the U.N. said Sunday that "an urgent investigation is needed and the international community must respond."

A spokesman for U.N. Secretary-General Antonio Guterres said the U.N. could not verify the attack, but noted "that any use of chemical weapons, if confirmed, is abhorrent, and requires a thorough investigation."

More than 100 buses entered the town of Douma Sunday night to transport fighters and their families to Jarablus, a town under the shared control of rebels and Turkey, said Syrian state-affiliated al-Ikhbariya TV.

The preparations follow a pattern of evacuations around the capital and other major Syrian cities as the government reasserts its control after seven years of war.

Human rights groups and U.N. officials say the tactic amounts to forced displacement, a war crime.

The Army of Islam could not be immediately reached for comment.

In his tweets Sunday, Mr. Trump called Assad an "animal" and delivered a rare personal criticism of Russian President Vladimir Putin for supporting him. A top White House aide, asked about the possibility of another U.S. missile strike, said, "I wouldn't take anything off the table."

The developments come as Mr. Trump has declared his intent to withdraw U.S. troops from Syria in the coming months despite resistance from many of his advisers.

Bakkar said several bombs laced with chemicals landed in Douma Saturday night. Another activist, Bilal Abou Salah, said a large, yellow cylinder smashed through the roof of an apartment building and came to rest on the third floor and started to discharge gas.

The Syrian Civil Defense group documented 42 fatalities but was impeded from searching further by strong odors that gave rescuers difficulties breathing, said Siraj Mahmoud, a spokesman for the group.

A joint statement by the civil defense group and the medical society said that more than 500 people, mostly women and children, were brought to medical centers complaining of difficulty breathing, foaming at the mouth and burning sensations in the eyes. Some had bluish skin, a sign of oxygen deprivation.

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

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

"Until this minute, no one has been able to find out the kind of agent that was used," Mahmoud said in a video statement from northern Syria.

The Syrian government, in a statement posted on the state-run news agency SANA, denied the allegations. It said the claims were "fabrications" by the Army of Islam and a "failed attempt" to impede government advances.

"The army, which is advancing rapidly and with determination, does not need to use any kind of chemical agents," the statement said.

The latest assault on Douma came after talks between the Army of Islam and Russia collapsed Friday, ending 10 days of calm for residents trapped inside.

Russia denied any involvement in the attack. Maj. Gen. Yuri Yevtushenko was quoted by Russian news agencies Sunday as saying Russia was prepared to send specialists to Douma to "confirm the fabricated nature" of the reports.

Douma has been crippled by close to five years of siege by government forces. It was once one of the hubs of the 2011 Arab Spring uprising against President Bashar Assad's government.

In recent weeks, government forces have recaptured villages and towns in the eastern Ghouta suburbs of the capital. Douma was the only town left holding out.

A 2013 chemical attack in eastern Ghouta that killed hundreds of people was widely blamed on government forces. The U.S. threatened military action but later backed down.

Syria denies ever using chemical weapons during the war and says it eliminated its chemical arsenal under a 2013 agreement brokered by the U.S. and Russia.

CBS News' Pamela Falk contributed reporting.