As images spread of children gasping and convulsing, aid groups and international leaders are denouncing the Syrian regime for their alleged role in a suspected chemical attack.
More than 70 people were reportedly killed Saturday in Douma, the last rebel-held town in Eastern Ghouta, the International Rescue Committee said Sunday.
"The evidence points toward yet another chemical attack by the regime," said Maja Kocijancic, spokeswoman for European Union Foreign Affairs and Security Policy.
"Almost a year to the day of the horrific attacks in Khan Sheikhoun, it is a matter of grave concern that chemical weapons continue to be used, especially on civilians."
The International Rescue Committee said the latest incident "only serves to further demonstrate that the government of Syria will stop at nothing to win this war."
But the Syrian government and its key ally, Russia, vehemently denied involvement in Saturday's attack. Instead, they accused rebels in Douma of fabricating the chemical assault claims in order to hinder the army's advances and provoke international military intervention.
Russia's Foreign Ministry called the reported attack a "hoax" that interfered with a deal to end fighting in Douma and evacuate civilians. Dozens of buses entered Douma on Sunday evening to take detainees released by Jaish al-Islam, the last remaining rebel group in Douma, to government-held territory, according to Syrian state TV. The vehicles will also transport rebels and civilians to northern Syria, state TV said.
Iran, another ally, also defended the Syrian regime.
"The Islamic Republic of Iran strongly condemns the use of such (chemical) weapons by any party and anywhere in the world," Foreign Ministry spokesman Bahram Qasemi told the state-run Islamic Republic News agency.
He said the allegation that the Syrian regime is behind the attack "is not compatible with reality."
But anti-government activists claimed Syrian military helicopters dropped barrel bombs filled with chemicals on the town, suffocating some residents and sending others into violent convulsions.
Graphic footage shot by rescuers and activists show victims -- including children -- dead and injured, some ghostly white and foaming at the mouth in makeshift clinics. Others were found suffocated in their homes, according to first responders.
At least 48 people died and 500 others displayed symptoms similar to exposure to "toxic gas" in the Douma area, the White Helmets rescue group and the Syrian American Medical Society charity group said in a joint statement.
Other groups have announced varying death tolls in the wake of the attack. CNN could not independently verify the authenticity of the footage or the reports.
US President Donald Trump described the attack as "SICK" and criticized Russian President Vladimir Putin and Iran for supporting Syrian President Bashar al-Assad. He threatened that there would be a "big price to pay" for the attacks.
Saturday's attack occurred almost a year to the day after the United States struck a Syrian airfield in response to a chemical attack on the rebel-held town of Khan Sheikhoun.
The State Department described the incident in Douma as "horrifying" and said that if the use of chemical agents in the attack was confirmed, it would "demand an immediate response by the international community."
The US, United Kingdom, France, Poland, the Netherlands, Sweden, Kuwait, Peru and Cote d'Ivoire will participate Monday in an emergency UN Security Council meeting, the US mission to the United Nations said.
"The Security Council has to come together and demand immediate access for first responders, support an independent investigation into what happened, and hold accountable those responsible for this atrocious act," US Ambassador to the United Nations Nikki Haley said in a statement.
Following the attack on Saturday night, doctors in Eastern Ghouta saw patients shaking uncontrollably and some who appeared to be paralyzed and unresponsive, an official from the Union of Medical Care and Relief Organizations (UOSSM) told CNN.
The official, who asked to be identified as Dr. Jad, is in touch with local doctors and said one of the affected areas was the residential area of Masaken, where hundreds of civilians reside in underground shelters.
State TV: Deal reached to evacuate Douma rebels
The attack comes as Syrian forces are on the verge of reclaiming the last rebel-held areas in the country. Douma is the last town held by rebels in Eastern Ghouta, which was besieged for six years and had been heavily bombarded since mid-February.
On Sunday, Syrian state TV reported that the government had reached an agreement with Jaish al-Islam, the last remaining rebel group in Douma, to leave the enclave in the next 48 hours.
As part of the agreement, the group's fighters would be transported to Jarablus in northern Syria. In exchange, the rebels would release all prisoners they are holding in Douma.
Jaish Al-Islam didn't immediately respond to CNN's request for confirmation.
Talks between the rebel group and Russia collapsed on Friday. The Syrian government later resumed airstrikes in the rebel-held town, killing scores of people. Rebels responded with mortar attacks on Damascus, killing at least 12 people.
Sources close to the Syrian army told CNN that the military had advanced nearly a kilometer into the Douma area on Saturday.
Hundreds of people have been killed and thousands more wounded in the offensive on Eastern Ghouta, which was once home to an estimated 400,000.
Around 130,000 people have left the enclave in the past month, according to the United Nations. Of these, 83,000 have gone to eight collective shelters in government-controlled areas on the outskirts of Damascus.
Many have also fled to Idlib in the northwest, the largest remaining rebel-held area in the country.
Turkey, which earlier this year launched its own military offensive against Kurdish groups in Afrin, northern Syria, said in a statement Sunday that countries with leverage over the Syrian regime had an obligation to help "prevent future war crimes and crimes against humanity in Syria."
Underground weapons factory?
The Syrian regime has long accused rebel groups of launching chemical attacks in the country, and last week the Syrian military took CNN to what it claimed was an underground weapons factory belonging to rebels in Eastern Ghouta. Inside they showed off chemicals, fuses and mortar casings they said were used by rebels to manufacture weapons.
Government officials also showed CNN a handwritten manual detailing instructions for how to build incendiary weapons, including white phosphorus munitions, amongst other things. Officials say the manual was left behind by the rebels.
At another site, the military showed an underground storage facility they say belonged to the rebels inside a civilian area. The facility included an SA-5 surface-to-air missile.
CNN could not independently verify these claims.
One year after Khan Sheikhoun
The Syrian regime has been accused many times of turning chemical weapons on its people over the course of the war.
In April 2017, more than 80 people were killed in a sarin gas attack on the rebel-held town of Khan Sheikhoun.
That attack prompted the United States to launch 59 Tomahawk cruise missiles at a Syrian airbase.
A joint report from the United Nations and international chemical weapons inspectors last October determined that Assad's government was responsible for the attack. Damascus denied it was behind the attack and has repeatedly denied it has any chemical weapons.
Saturday's attack comes amid uncertainty about what role, if any, the US will play in Syria in the future.
The US has about 2,000 troops in Syria, where they advise local forces fighting ISIS. President Trump has said he wants to bring American troops home, but last week agreed to keep them in Syria for the short-term to help defeat the terror group.