However, the Britain-based monitoring group was unable to confirm the nature of the substance.
Beirut: At least 58 civilians, including two children, were killed in an air strike early on Tuesday that released "toxic gas" on a town in northwest Syria, a monitor said.
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. It said that the dead include 11 children and says the toll is likely to climb further because of the large number of injured.
The Britain-based monitoring group was unable to confirm the nature of the substance, however.
Syria's opposition National Coalition accused President Bashar al-Assad's government of the 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.
Syrian opposition activists described the attack as among the worst poison gas attacks in the country's six-year civil war. They claimed it was caused by an airstrike carried out either by the Syrian government or Russian warplanes.
There was no immediate comment by Syrian or Russian officials or any international agency on the attack.