President Trump lashed out Sunday against an alleged chemical weapons attack by Syrian government forces on civilians, ripping "that animal Assad" and laying blame for the Syrian president's power on Russia, Iran and even President Barack Obama.

Trump's tweet storm came hours after the White Helmets, a civil defense force in rebel-held areas of Syria, claimed that entire families were gassed to death Saturday night in Douma and East Ghouta. The group, which put the death toll at more than 40, said many residents were hiding in cellars, suffocating from poison gas.

Syrian state media said Sunday that rebel forces led by the Army of Islam had agreed to leave Douma within 48 hours as Syrian leader Bashar Assad tightened his grip on rebel strongholds around Damascus.

"Many dead, including women and children, in mindless CHEMICAL attack in Syria," Trump said on Twitter. "Area of atrocity is in lockdown and encircled by Syrian Army, making it completely inaccessible to outside world."

The attack came less than a week after Trump, speaking about Syria, declared "I want to bring our troops back home." A day later, however, the White House signaled that a U.S. withdrawal from Syria is not imminent.

On Twitter on Sunday, Trump demanded that the area be opened up for medical help and verification of a chemical attack, which would constitute a war crime. He called the attack another "humanitarian disaster for no reason whatsoever. SICK!"

Trump blamed Russian President Vladimir Putin and Iran for providing support to Assad. Trump also blamed Obama for allowing Assad to cross his predecessor's "state Red Line in the Sand." If Obama had acted years ago, Trump said, the Syrian crisis would have ended long ago and Assad would have passed into history.

The attack comes a year to the day after Trump authorized a military strike on a Syrian government airfield after a sarin gas attack that killed at least 85 people in Khan Sheykhun.

Russia on Sunday said allegations of a chemical attack are a ruse aimed at justifying military strikes by foreign forces and warned of grave consequences in case of any military retaliation.

"It is necessary to once again caution that military intervention under false and fabricated pretexts in Syria, where the Russian servicemen stay at the request of the legitimate government, is absolutely unacceptable and may trigger the gravest consequences," Russia said on its U.S. embassy's Facebook page.

The White Helmets said more than 500 people, most of them women and children, were brought to local medical centers with symptoms consistent of exposure to a chemical agent. Patients showed signs of respiratory distress, burned eyes, foaming of the mouth and other symptoms that included "emission of chlorine-like odor," the group said.

The reports could not immediately be verified independently. Syria called the claims "fabrications," and Iran and Russia also denied that any chemical weapons were used.

Bahram Qasemi, spokesman for the Iran Foreign Ministry, said the claims were an excuse for U.S. military action. Maj. Gen. Yuri Yevtushenko, who heads Russia's center for Syrian reconciliation, blamed the allegations on White Helmets' "fake news."

"We strongly reject this information and confirm readiness after Douma is liberated from militants to send Russian specialists in radiation, chemical and biological protection to collect data to confirm that these statements are fabricated," Yevtushenko said.

The civil war has been raging in Syria for seven years. The toll has been horrifying: The U.N. estimates more than 400,000 people have died. Another 5 million have fled the country, and 6 million more have abandoned their homes but remain within the beleaguered nation of less than 20 million people.

Syrians are no strangers to chemical weapons. An attack in Syria's Idlib province by government forces a year ago killed almost 100 people, including women and children. An investigation by the international watchdog Organization for the Prohibition of Chemical Weapons confirmed that sarin nerve gas was used on the town of Khan Sheikhoun.

Photos and video of the aftermath that included children dying on camera sparked global outrage directed at the Assad regime. Trump cited the images when he launched cruise missiles on a Syrian-controlled air base.

Fred Hof, a senior fellow at the Atlantic Council, said that the use of chemical agents again would be a "signal of utter contempt" for the U.S.

“Assad may be interpreting (Trump’s) recent statements about the United States 'getting out of Syria' as a green light to do what he wants,” Hof said. He also warned that Obama’s "erasure" of the chemical weapons red line in 2013 had destabilizing implications — including damage to America’s credibility — far beyond Syria. Hof said Trump can either erase his own red line or take action, as he did in 2017.

British Foreign Secretary Boris Johnson called the chemical weapons report "grim." The European Union called it a matter of "grave concern" and called on Syria, Russia and Iran to prevent further attacks.

The Syrian Women's Political Movement on Sunday urged the international community to end its "blatant complacency." The group issued a statement calling for the U.N. to send in teams to investigate Saturday's attack and to treat the victims.

"The ongoing silence of the international community and its failure to protect civilians in Syria is a disgrace for humanity," the statement said. "One that history will remember."