"As far as French intelligence services and military intelligence are concerned, in a matter of days we will prove that the regime has indeed organized these strikes with chemical weapons", Ayrault said.
Ahmet Uzumcu, director-general of the Organization for the Prohibition of Chemical Weapons, said Wednesday that the results "from four OPCW designated laboratories indicate exposure to sarin or a sarin-like substance".
When the Syrian town of Khan Sheikhoun was hit this month by poison gas, the attack was condemned internationally and prompted the Trump administration to launch a retaliatory missile strike.
The attack on the rebel-held town killed at least 87 people.
After the USA strike, Russian Federation said it would suspend its participation in a hotline with the United States military to "deconflict" or minimize the risk of mid-air collisions in Syrian airspace, but US officials said this week that the two militaries continued to communicate.
The global chemical arms watchdog Thursday "overwhelmingly" rejected a Russian-Iranian move to launch a new investigation into a suspected chemical attack in Syria, delegates said, backing the probe already underway. Israel welcomed the strike on its northern neighbor.
Brigadier-General Zaher al-Sakat, who served as the head of chemical warfare in a top Syrian military unit before defecting in 2013, said that Assad had not declared large amounts of sarin and its precursor chemicals.
"We have elements that will enable us to show that the regime knowingly used chemical weapons", he added. The government of President Bashar Assad has denied responsibility and blamed opposition forces.
The Syrian regime had agreed to surrender its chemical weapons arsenal in 2013, in a bid to hold off U.S. intervention in retaliation for a chemical attack on an opposition-held Damascus suburb earlier that year which killed hundreds and prompted worldwide outrage.
UK-based chemical weapons expert Dan Kaszeta told the Associated Press that Israel's estimate was conservative, but nonetheless was enough to be highly lethal. "It could also be used for roughly 10 attacks of a similar size to the recent Khan Sheikhoun attack".
The UN said sarin had been used in that attack, which killed hundreds of people in the Ghouta agricultural belt to the west and east of the capital.
The finding supported earlier testing by Turkish and British laboratories.
The aircraft in question reportedly were moved to an airfield close to a base where the majority of Russian air forces helping the Assad regime are located.