The Britain-based Syrian Observatory for Human Rights, an opposition monitoring group, put the number of dead at 58, although unconfirmed reports said over 100 were killed.
The nature of the substance has yet to be confirmed, but a British doctor at the scene tweeted that the chemical released was sarin gas.
The Syrian Coalition, an opposition group based outside the country, said government planes carried out the airstrike on Khan Sheikhoun, south of the city of Idlib, the provincial capital. Rights groups have long accused President Bashar Assad of deploying chemical weapons against his own citizens.
The Syrian Observatory for Human Rights confirmed the deaths of 11 children from chemical exposure, but footage posted on Twitter showed some 20 young children lying motionless in a room.
Hours after the attack, a rocket slammed into a hospital as doctors treated victims, destroying part of the building, an AFP correspondent said. The projectile brought down rubble on top of medics as they struggled to deal with victims of the attack. READ MORE