France has said it will provide evidence within days that the Syrian regime was behind the chemical attack which killed more than 80 people, including up to 30 children.
The gas attack in Khan Sheikhoun on 4 April, which has been widely blamed on Syria's president Bashar al Assad, prompted the US to retaliate with a cruise missile strike on one of the country's airbases.
Jean-Marc Ayrault, France's foreign minister, told French TV: "We will provide proof that the regime did indeed organise these strikes with chemical weapons."
He said an analysis of the attack was under way, adding: "In a few days, I will be able to provide proof."
The Syrian regime and its backers, including Russia, have denied Mr Assad was responsible.
They have said the attack was either a rebel provocation or the result of Syrian planes hitting a rebel weapons factory.
Mr Assad last week questioned whether the attack happened at all, saying there are "a lot of fake videos now".
"We don't know whether those dead children were killed in Khan Sheikhoun," he said in an interview with AFP TV. "Were they dead at all?
"Who committed the attack, if there was an attack? What's the material? You have no information at all, nothing at all. No one investigated."
Russia and Iran have issued a joint call for a new "full-scale and thorough investigation" into the attack.
The Organisation for the Prohibition of Chemical Weapons (OPCW) has said "incontrovertible" test results by its team of experts already probing the incident had shown sarin gas or a similar substance was used.
The governing body of the global chemical arms watchdog is expected to vote on a fresh investigation.
Moscow has criticised the OPCW for not sending experts to the attack site, saying it was "unacceptable to analyse events from a distance".
But OPCW director general Ahmet Uzumcu said a team was ready to head to the town "should the security situation so permit".
"I am told that this would require a 48-hour ceasefire and safe passage for the team to be arranged," he said.
Russia last week vetoed a UN Security Council resolution demanding a probe, prompting UK Foreign Secretary Boris Johnson to say Moscow had put itself "on the wrong side of the argument".
Matthew Rycroft, Britain's ambassador to the UN, also said UK analysis of samples from the scene of the attack in Idlib province had detected sarin or a sarin-like substance.