syria chemical
Syrian children carry placards - which reads "We didn't forget the memory of the massacre, Ghouta, Damascus countryside Syria, 1466 martyrs in the chemical attacks, the massacre continues" - in front of the offices of the U.N. headquarters in Beirut, Dec. 26, 2015. REUTERS/Jamal Saidi

Update: 6:24 a.m. EDT — A hospital that was helping treat victims of Tuesday’s “chemical attack” in the rebel-held town of Khan Sheikhoun, in Syria’s Idlib province was bombed soon after, the country’s main opposition said on Twitter.

Original story:

Dozens of people were killed in a suspected chemical attack on a rebel-held town in northwestern Syria, numerous reports said Tuesday.

According to UK-based Syrian Observatory for Human Rights, at least 35 people were killed, several of whom were children, as warplanes allegedly carried out an attack on the rebel-held town of Khan Sheikhoun, in Idlib province, with a toxic gas. Other outlets put the death toll much higher, with more casualties expected as hospitals filled up with the injured.

Read: Global Watchdog Looks To Prevent Chemical Attacks From Terror Groups In Syria

The casualties were reported to be from the effect of the gas and a large number of people were said to be suffering from respiratory problems. According to the Associated Press, pictures and videos from the scene showed civilians limping, while some were also reportedly foaming at the mouth.

The nature of the substance has not been confirmed. The monitoring group was also unable to determine whether the planes belonged to the Syrian regime of President Bashar Assad or its ally Russia.

The Syria Civil Defense search-and-rescue group, popularly known as the White Helmets, did not release a death toll but released photos of paramedics washing down victims. Other media outlets also published videos of victims stripped down to their underwear while many remained unresponsive.

In a post on Twitter, local doctor Shajul Islam said the hospital he worked at was receiving a large number of patients, many of whom seemed to be showing symptoms of exposure to sarin, a chemical weapon that is extremely potent as a nerve agent. He also posted a video of his patients.

The attack comes days after Assad’s forces were accused of using chemical weapons in a counter-offensive in the nearby Hama province which left over 50 people with severe respiratory problems. Since the start of the civil war in the country in 2011, Syria has seen over 320,000 people die in the conflict, Agence France-Presse reported.

After a United States-Russia agreement in 2013, the Organisation for the Prohibition of Chemical Weapons was tasked with the destruction of all chemical weapons declared by Syria. According to OPCW, the destruction of the stockpile was completed in 2016.