The use of chemical weapons in Syria is a red line for France and would result in reprisals, President Emmanuel Macron said after meeting his Russian counterpart Vladimir Putin.
Meeting Mr Putin for the first time, Mr Macron told a news conference that France and Russia must cooperate to "eradicate terrorist groups" in Syria and did not directly criticise Moscow's role there.
France and Russia back different sides in the Syrian conflict, with Mr Putin supporting President Bashar al-Assad and Mr Macron part of a western coalition that supports rebel groups and has accused Mr Assad of using chemical weapons in the past.
Mr Macron said it was essential to talk with all actors in the Syria conflict, including representatives of Mr Assad.
"Our two countries will cooperate on Syria, this is essential," Mr Macron said.
"We need strong co-operation because we have a joint priority, which is the fight against terrorism."
Mr Macron said he wanted Paris and Moscow to bolster intelligence sharing on Syria and to work together on finding a political solution to the conflict, but gave no details on what a political deal might look like.
Sounding less forthcoming, Mr Putin said he was not sure if France's Syria policy was "independent" because it was part of a US-lead alliance, adding that Paris and Moscow had points of disagreement and agreement over Syria.
Mr Putin said he and Mr Macron had agreed the fight against terrorism was their top priority, but stressed that he had not changed his views on Syria and told Mr Macron so.
Mr Macron's warning of French retaliation in the event chemical weapons are used echo the line taken by US President Donald Trump, who in April ordered cruise missile strikes in retaliation for a chemical weapons attack that Washington blamed on Damascus.
"Any use of chemical weapons would results in reprisals and an immediate riposte, at least where France is concerned," Mr Macron said, standing next to Mr Putin in the Versailles palace outside of Paris.
French intelligence concluded that forces loyal to President Assad carried out the attack in northern Syria and that Assad or members of his inner circle ordered the strike, a declassified French report showed.
Mr Assad has denied his government used chemical weapons.
