Updated at 21:27
At least 100 people died in a suspected gas attack in Syria's Idlib province, according to a Syrian medical relief group.
UOSSM also said that 400 people had been injured.
People choked or fainted after the attack, while some were seen foaming at the mouth, the Syrian Observatory for Human Rights said.
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.
The Syrian military has denied involvement in the attack, and says it would never use chemical weapons.
It was not immediately clear if the deaths had been caused by the chemical weapons or injuries sustained in 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.
#UPDATE Syria 'chemical attack' kills dozens, sparks widespread outrage https://t.co/1PzLROvFoZ pic.twitter.com/RmGgswijB4
— AFP news agency (@AFP) April 4, 2017
Rescue workers were pictured hosing down children. More than 60 people were reportedly injured in the airstrikes.
France asked for an emergency United Nations Security Council meeting in response to the reports from Idlib, and AFP reports the council will meet tomorrow.
In comments quoted by CNN, White House Press Secretary Sean Spicer said: "Today's chemical attack in Syria against innocent people including women and children is reprehensible and cannot be ignored by the civilised world."
"They were unconscious, they had seizures"
Mohammed Hassoun, an activist in nearby Sarmin, said 18 cases had been taken to a hospital there.
"Because of the number of wounded, they have been distributed around in rural Idlib," he said.
"They were unconscious, they had seizures and when oxygen was administered, they bled from the nose and mouth."
Russia's defence ministry said it had not carried out any airstrikes in the area.
The opposition National Coalition also 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.
