At least 58 people, including 11 children, have been killed and dozens wounded in a suspected chemical attack in rebel-held Syrian town of Idlib, the Syrian Observatory for Human Rights says.
The monitoring group reported that strikes on Khan Sheikhoun by Syrian government or Russian jets had caused many people to choke.
Later, aircraft fired rockets at local clinics treating survivors, doctors and activists said.
A Syrian military source denied the government had used any such weapons.
Russia’s defense ministry meanwhile insisted it had not carried out any air strikes in the vicinity.
If confirmed, it would be one of the deadliest chemical attacks in Syria’s civil war.
The warplanes are reported to have attacked Khan Sheikhoun, about 30 miles south of Idlib, on April 4, when many people were asleep.
Hussein Kayal, a photographer for the pro-opposition Edlib Media Center (EMC), told the Associated Press that he was awoken by the sound of an explosion at about 06:30.
When he reached the scene, there was no smell, he said. He found people lying on the floor, unable to move and with constricted pupils, he added.
The Syrian Observatory (SOHR) quoted doctors as saying that they had been treating people with symptoms including fainting, vomiting and foaming at the mouth.
An AFP journalist saw a young girl, a woman and two elderly people dead at a hospital, all with foam still visible around their mouths.
The journalist also reported that the same facility was hit by a rocket on April 4, bringing down rubble on top of doctors treating the injured.
The source of the projectile was not clear, but the EMC and the opposition Local Co-ordination Committees network said warplanes had targeted several clinics.
The SOHR put the death toll at 58, including 11 children, but the head of a charity ambulance service in Idlib, Mohammed Rasoul, said that 67 people had been killed and that 300 were injured.
The pro-opposition Step news agency meanwhile said 100 had died.
Sarin inhibits the action of an enzyme, which deactivates signals that cause human nerve cells to fire. This blockage pushes nerves into a continual “on” state. The heart and other muscles – including those involved in breathing – spasm.
Sufficient exposure to Sarin can lead to death via asphyxiation within minutes.
The substance is almost impossible to detect because it is a clear, colorless and tasteless liquid that has no odor in its purest form.
The Syrian government was accused by Western powers of firing rockets filled with Sarin at several rebel-held suburbs of the capital Damascus in August 2013, killing hundreds of people.
President Bashar al-Assad denied the charge, blaming rebel fighters, but he did subsequently agree to destroy Syria’s chemical arsenal.
Despite that, the Organization for the Prohibition of Chemical Weapons has continued to document the use of toxic chemicals in attacks in Syria.
In January 2016, the organization said blood samples taken from the victims of one unspecified attack showed victims had been exposed to Sarin or a Sarin-like substance.
A joint investigation with the UN concluded in October that Syrian government forces had used chlorine as a weapon at least three times between 2014 and 2015.
It also found ISIS militants had used the blister agent sulphur mustard.
Human Rights Watch also recently accused government helicopters of dropping bombs containing chlorine on rebel-held areas of Aleppo on at least eight occasions between November 17 and December 13, during the final stages of the battle for the city.
Idlib province, where the air strikes took place, is almost entirely controlled by a rebel alliance and the al-Qaeda-linked jihadist group, Hayat Tahrir al-Sham.
The region, home to 900,000 displaced people, is regularly targeted by the government and its ally Russia, as well as the US-led coalition against ISIS.
There was no immediate comment from the government, but a Syrian military source told Reuters that it “does not and has not” used chemical weapons.