At least 58 people, including 11 children, died in a suspected gas attack in Syria's Idlib province, according to activists.
People choked or fainted after the attack, while some had foam coming out of their mouths, the Syrian Observatory for Human Rights,
The UK-based group opposed to the Syrian government said it had received the reports from medics on the ground in the town of Khan Sheikhoun.
The Syrian Coalition, an opposition group also based outside the country, said planes from President Bashar al Assad's military carried out the airstrikes.
Videos purporting to show the aftermath circulated on social media.
One showed the bodies of several young children being covered with a blanket, while another showed men lifting a body into the back of a truck.
In a number of videos, medics could be seen helping people who appeared to have breathing difficulties.
Rescue workers were pictured hosing down children.
More than 60 people were also reportedly injured in the airstrikes.
The Syrian government did not immediately comment on the allegations, but last week said claims the government was using chemical weapons were "devoid of truth".
The opposition National Coalition called for an emergency session of the UN Security Council, blaming the airstrikes on the "regime of the criminal Bashar".
It urged the UN to "open an immediate investigation and take the necessary measures to ensure the officials, perpetrators and supporters are held accountable".
"Failure to do so will be understood as a message of blessing to the regime for its actions," it added.
Idlib province is almost entirely controlled by the Syrian opposition and is home to 900,000 people displaced by the war.