Syrian rebel groups accused the government of dropping a barrel bomb full of poisonous chemicals in eastern Ghouta yesterday.
The US has called on Russia to end its support for the Syrian regime after the horror attack in the town of Douma.
State Department spokeswoman Heather Nauert said: “These reports, if confirmed, are horrifying and demand an immediate response by the international community.
“Russia, with its unwavering support for the regime ultimately bears responsibility for these brutal attacks.”
Syrian state media denied government forces had launched any chemical attack and accused rebel forces of spreading fake news.
Volunteer rescue team The White Helmets — who first estimated up to 150 had been killed — tweeted graphic images showing bodies of men, women and children hiding in a basement.
Another video circulated by activists shows lifeless bodies with foam in their mouths as a voice says: “Douma city, April 7 … there is a strong smell here”.
Many of the images are so graphic and disturbing that The Sun Online has chosen not to publish them.
One upsetting photograph shows a slain family hiding in a basement as the dad shields his wife and baby.
Relief groups said a chlorine bomb hit a hospital in the Syrian city of Douma killing six people — and a second bomb hit a nearby apartment building, killing at least 35.
The Syrian Observatory for Human Rights said 11 people had died as a result of suffocation caused by the smoke from conventional weapons being dropped by the government.
Rami Abdulrahman, the Observatory director, said he could not yet confirm if chemical weapons had been used.
Basel Termanini, the U.S.-based vice president of volunteer medics SAMS, said: “We are contacting the U.N. and the U.S. government and the European governments.”
A joint statement from SAMS and the White Helmets this morning said 49 people had died.
They added that medics had treated more than 500 people with breathing difficulties, frothing from the mouth and smelling of chlorine.
Syrian state news agency SANA said the rebel group in Douma, Jaish al-Islam, was making “chemical attack fabrications in an exposed and failed attempt to obstruct advances by the Syrian Arab army.
The Syrian government has repeatedly denied using chemical weapons.
Syrian government forces resumed their offensive on rebel-held Douma on Friday afternoon after a ten-day truce collapsed.
Violence resumed days after hundreds of opposition fighters and their relatives left Douma toward rebel-held areas in northern Syria.
Douma is the last rebel stronghold in eastern Ghouta and has been the government offensive there has been the deadliest of the seven-year conflict.
More than 1,600 civilians have been killed there according to the Syrian Observatory for Human Rights.