Air strikes hit parts of Syria's rebel-held Idlib province on Friday, a war monitor said, the same day as the presidents of Iran, Russia and Turkey will meet in Tehran to discuss what to do about the enclave.

The Syrian Observatory for Human Rights said the strikes were targeting positions belonging to rebel groups in the northern Hama and southern Idlib provinces.

Around 3 million people live in the last major stronghold of active opposition to President Bashar al-Assad's rule, which comprises most of Idlib province and adjacent small parts of Latakia, Hama and Aleppo provinces.

Damascus, backed by allies Russia and Iran, has been preparing an assault to recover those rebel-held parts of the northwest, and resumed air strikes alongside Russia on Tuesday after weeks of lull.

The Britain-based Observatory said strikes on Friday had destroyed a building used by the Islamist Ahrar al-Sham group near the town of al-Habeet resulting in a number of casualties.

Ahrar al-Sham is part of the Turkey-backed National Liberation Front alliance which formed earlier this year.

Russian officials have stated clearly they want militants to be pushed out of Idlib. The United Nations has warned that a military offensive in Idlib could cause a humanitarian catastrophe.