Turkey has said it is working with Russia and Iran to stabilize Syria's rebel-held Idlib region, indicating continued efforts to avoid a Syrian government offensive in the area, where Ankara failed to secure a cease-fire last week.

Damascus, backed by Russia and Iran, has been preparing a major assault to recover the region. The United Nations has warned this could trigger a humanitarian catastrophe in an area where 3 million people live.

Idlib is part of an arc of territory at the Turkish border representing the last major area held by insurgents who have fought President Bashar Assad through more than seven years of war. Assad has vowed to recover "every inch" of Syria.