A Syrian child receives treatment following a suspected toxic gas attack in Khan Sheikhun, a rebel-held town in the northwestern Syrian Idlib province, on April 4, 2017.

At least 100 people, including children, were killed in an air strike early Tuesday that released “toxic gas” on a town in northwest Syria, Idlib health officials said.

At least 400 people are reported injured.

The Syrian Observatory for Human Rights said those killed in the town of Khan Sheikhun, in the central province of Idlib, had died from suffocation and the effects of the gas.

Syria’s opposition National Coalition accused President Bashar al-Assad’s government of the suspected toxic gas attack and demanded a UN investigation.

“The National Coalition demands the Security Council convene an emergency session…, open an immediate investigation and take the necessary measures to ensure the officials, perpetrators and supporters are held accountable,” the body said in a statement.