DAMASCUS, Syria (CNN) -- Dozens of Syrians were killed and hundreds of others were affected by a suspected chemical attack on the last rebel-held town in Eastern Ghouta, rescue workers and an aid group said Sunday.
Anti-government activists claimed Syrian military helicopters dropped barrel bombs filled with chemicals on Douma, east of the capital Damascus, on Saturday night, suffocating some residents and sending others into violent convulsions.
Graphic footage shot by rescuers and activists show people -- including children -- dead and injured, some ghostly white and foaming at the mouth in makeshift medical centers. Others were found suffocated in their homes, according to first responders.
On Sunday the Syrian government and Russia, its key ally, vehemently denied involvement and accused rebels in Douma of fabricating the chemical attack claims in order to hinder the army's advances and provoke international military intervention.
US President Donald Trump has been briefed on the attack, a White House official told ABC News on Sunday. The State Department described the incident 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."
At least 48 people died and 500 others displayed symptoms similar to exposure to "toxic gas" in the Douma area on Saturday, said the White Helmets rescue group and the Syrian American Medical Society, a charity, in a joint statement on Sunday.
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.
Following the attack on Saturday night, doctors in Eastern Ghouta saw patients convulsing and some who appeared to be paralyzed and unresponsive, an official with the Union of Medical Care and Relief Organizations (UOSSM) told CNN.
The official, who asked to be identified as Dr. Jad and is in touch with local doctors, said one of the affected areas was the residential area of Masaken, where hundreds of civilians reside in underground shelters.
Syrian army poised to retake Douma
The attack comes as Syrian forces are on the verge of recapturing Douma, the last town held by rebels in Eastern Ghouta, following a brutal offensive launched in mid-February.
Sources close to the Syrian army told CNN that the military had advanced nearly a kilometer into the Douma area on Saturday.
On Sunday the government said it was negotiating with Jaish al-Islam, the last remaining rebel group in Douma, to evacuate its fighters from the town to Jarablus in northern Syria, the state-run news agency SANA reported.
Talks between Russia and the rebel group 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.
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.
Underground weapons factory?
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 Syrian President Bashar al-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 approximately 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.
Tamara Qiblawi and Fred Pleitgen reported from Damascus, Sheena McKenzie wrote in London. Steve Almasy and Milena Veselinovic contributed to this report.
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