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 (LCC) network said warplanes had targeted several clinics.
Pro-government journalists later cited military sources as saying there had been an explosion at an al-Qaeda chemical weapons factory in Khan Sheikhoun that was caused either by an air strike or an accident.
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.
One aid agency, the Union of Medical Care and Relief Organisations (UOSSM), put the death toll at more than 100 and said that the figure was likely to rise.
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.
Chemical weapons expert Dan Kaszeta said that determining whether Sarin was involved simply by examining video clips is problematic.
He added that Tuesday's attack could have been the result of one of any number of chemical agents as they tend to "behave the same in terms of their physiological effects on the human body".
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 (OPCW) 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.
Other reported use of 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
What has been the reaction to Tuesday's attack?
UN Syria envoy Staffan de Mistura said it was an "horrific" attack and that there should be a "clear identification of responsibilities and accountability" for the attack on the rebel-held town.
French President Francois Hollande accused the Syrian regime of a "massacre". "Once again the Syrian regime will deny the evidence of its responsibility for this massacre," Mr Hollande said in a statement.
UK Foreign Secretary Boris Johnson said that President Bashar al-Assad would be guilty of a war crime if it were proved his regime was responsible.
"Bombing your own civilians with chemical weapons is unquestionably a war crime and they must be held to account," he said.
The UK and France, which said that reports suggested it was a "particularly serious chemical attack", have called for an emergency meeting of the UN Security Council.
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 OPCW said it was "seriously concerned" about the alleged chemical attack, adding that it was "gathering and analysing information from all available sources".