At least 58 people have been killed and dozens wounded in a suspected chemical attack on a rebel-held town in north-western Syria, a monitoring group says.
The Syrian Observatory for Human Rights 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, medics and activists said.
A Syrian military source denied the government had used any such weapons.
Russia's defence 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 50km (30 miles) south of the city of Idlib, early on Tuesday, 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 (03:30 GMT).
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.
Mohammed Rasoul, the head of a charity ambulance service in Idlib, told the BBC that his medics had found people, many of them children, choking in the street.
The Syrian Observatory (SOHR) quoted medics as saying that they had been treating people with symptoms including fainting, vomiting and foaming at the mouth.
An AFP news agency 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 Tuesday afternoon, 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.
How many victims are there?
The SOHR put the death toll at 58, including 11 children, but Mr Rasoul reported that 67 people had been killed and that 300 were injured.
The pro-opposition Step news agency meanwhile said 100 had died.
The EMC said it had stopped counting the victims because there were so many.
It posted photos showing what appeared to be at least seven dead children in the back of a pick-up truck. There were no visible traumatic injuries on their bodies.
What substance was used?
The SOHR said it was unable to say what exactly was dropped.
However, the EMC and LCC said it was believed to be the nerve agent Sarin, which is highly toxic and considered 20 times as deadly as cyanide.
Sufficient exposure can lead to death via asphyxiation within minutes.
Sarin is almost impossible to detect because it is a clear, colourless and tasteless liquid that has no odour in its purest form.
Has Sarin been used in Syria before?
The 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 Organisation for the Prohibition of Chemical Weapons has continued to document the use of toxic chemicals in attacks in Syria.
In January 2016, it said blood samples taken from the victims of one unspecified attack showed victims had been exposed to Sarin or a Sarin-like substance.
What about other chemical agents?
A joint investigation with the UN concluded in October that government forces had used chlorine as a weapon at least three times between 2014 and 2015.
It also found Islamic State 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 17 November and 13 December, during the final stages of the battle for the city.
And last week, two suspected chemical attacks were reported in Hama province, in a rebel-held area not far from Khan Sheikhoun.
What has been the reaction to Tuesday's attack?
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 IS.
There was no immediate comment from the government, but a Syrian military source told Reuters news agency that it "does not and has not" used chemical weapons.
The opposition National Coalition accused the government of being behind what it called a "horrible crime", and called on the UN Security Council to open an immediate investigation and hold the perpetrators accountable.
France, which backs the opposition, agreed that a "particularly serious chemical attack" had taken place and called for an emergency Security Council meeting.