Russian claims that a release of chemicals that killed and injured dozens of civilians in northern Syria came from rebel weapons on the ground have been rejected.
Britain's foreign secretary, a rebel commander and a weapons expert all said the evidence pointed to an attack by Syrian government forces.
Damascus denies its forces launched a chemical weapons attack.
The issue will overshadow a conference on Syria in Brussels.
Seventy donor nations are discussing aid efforts in the war-ravaged country.
The UN Security Council is due to hold emergency talks later following the release of gas in Khan Sheikhoun in Idlib province which - according to UK-based monitoring group the Syrian Observatory for Human Rights - killed 72 people, including 20 children.
What happened?
Footage from Khan Sheikhoun following the incident on Tuesday showed civilians, many of them children, choking and foaming at the mouth.
Witnesses said clinics treating the injured were then targeted by air strikes.
Aftermath of attack in pictures The spectre of nerve agents in Syria - again
Some of the victims were treated across the border in Turkey. One woman in hospital said: "We were affected by the gas. We couldn't stand up. I felt dizzy and sick. I suffer from shortness of breath. I couldn't breathe."
Victims were treated for injuries, including asphyxiation
The Observatory cites reports from the area saying it was subjected to a further wave of air strikes on Wednesday.
What do the Russians say?
Russia, which backs Syrian President Bashar al-Assad's government, acknowledged that Syrian planes had attacked Khan Sheikhoun.
But it said the aircraft had struck a depot producing mines filled with a poisonous substance, for use by militants in Iraq.
"Yesterday [Tuesday], from 11:30am to 12:30pm local time, Syrian aviation made a strike on a large terrorist ammunition depot and a concentration of military hardware in the eastern outskirts of Khan Sheikhoun town," Russian defence ministry spokesman Igor Konoshenkov said.
"On the territory of the depot there were workshops which produced chemical warfare munitions."
He said the chemical munitions had been used by rebels in the city of Aleppo last year.
"The poisoning symptoms of the victims in Khan Sheikhoun shown on videos in social networks are the same as they were in autumn of the previous year in Aleppo," he added.
Local journalists say there are no military positions in the town itself, but an array of broadly aligned rebel groups controlling the area surrounding it.
The response
UK Foreign Secretary Boris Johnson responded to the Russian statement by saying: "All the evidence have seen suggests this was the [Syrian President Bashar] Assad regime... using illegal weapons on their own people."
A Syrian rebel commander described the Russian statement as a "lie".
Hasan Haj Ali, commander of the Free Idlib Army rebel group, told Reuters news agency: "Everyone saw the plane while it was bombing with gas.
"Likewise, all the civilians in the area know that there are no military positions there... The various factions of the opposition are not capable of producing these substances."
A chemical weapons expert, Colonel Hamish de Bretton-Gordon, told the BBC that the Russian version of events was "pretty fanciful".
The idea that a nerve gas like Sarin could spread after a weapons manufacturing process had been bombed was "unsustainable", he added.
International moves
Wednesday's emergency meeting of the UN Security Council was called by France and the UK as international outrage mounted over the incident.
US blames Assad over 'chemical attack'
Britain's ambassador to the UN, Matthew Rycroft, said the incident was "very bad news for peace in Syria".
"This is clearly a war crime and I call on the Security Council members who have previously used their vetoes to defend the indefensible to change their course," he told reporters in New York.
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