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Ankara and Moscow are again facing an escalation of violence in Syria's last rebel-held territory, a development that puts their cooperation to the test even as they support opposing sides in the eight-year war that has devastated Syria.

An all-out offensive by Syrian government forces to capture Idlib in northwestern Syria from insurgents could unleash an unprecedented humanitarian crisis, for the area is home to 3 million people.

Turkey, which is already hosting more than 3.6 million Syrian refugees, is facing strong pressure from Syria, Iran and Russia to deliver on its pledge to control the armed rebel factions in Idlib.

But Turkey also needs Russia to rein in Syrian President Bashar Assad to prevent a massive outflow of refugees and to keep Turkish soldiers on the ground safe.