French President Emmanuel Macron has called for Turkey and Russia to implement a lasting ceasefire in Syria's Idlib province in conversations with the two countries' leaders, the Elysee said.
Moscow-backed Syrian forces have since December led a military offensive against the final major rebel stronghold, where Ankara supports some rebel groups.
Macron expressed his "very strong concern about the unfolding humanitarian crisis" to Turkish president Recep Tayyip Erdogan and Russian leader Vladimir Putin, according to a statement released on Saturday.
He also warned of the risk terrorist groups would spread "because of the military offensive of the Syrian regime and its allies," adding it undermined the 2018 Idlib agreement between Russia and Turkey to create a demilitarised zone in the northwestern province.
The accord has fallen apart as Syrian president Bashar al-Assad's forces moved to recapture the last big region outside his control.
Macron said an "immediate halt to hostilities" is needed and called on Russia and Turkey to implement a "lasting and verifiable" ceasefire as outlined in that agreement.
Russia must "end its military offensive in northwest Syria and respect international humanitarian law, the protection of civilians, personnel and humanitarian access", he added.
Macron and German Chancellor Angela Merkel have called for a summit with Erdogan and Putin to seek an end to the crisis.