Turkey and Russia face conflicts over Syria’s push into Idlib province.

Countries back opposing sides in rebel-held area

ISTANBUL — 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. 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.

Turkish President Recep Tayyip Erdogan and Russian President Vladimir Putin “have an incentive to cooperate and ensure that nobody’s interests are totally trampled,” says Aaron Stein, the director of the Middle East program in American think-tank Foreign Policy Research Institute.

In September, the two leaders brokered a cease-fire for Idlib in the Russian resort of Sochi, preventing a bloody onslaught, despite the fact that Russia has firmly backed Assad and Turkey supports opposition forces. Nine months later, the truce has failed.

The agreement called for a 9-to12-mile demilitarized zone free of insurgents and heavy weaponry and for two key highways crossing through Idlib to be reopened. The demilitarized zone has been breached and the highways are at the center of the current government offensive.

Syrian ground forces have been advancing from the south of the rebel stronghold under the cover of Syrian and Russian airstrikes.

The Syrian Observatory for Human Rights said more than 291 civilians and 369 fighters have been killed since April 30 in the rebel stronghold. In the same period, 269 government troops and 22 civilians were killed in government areas by rebel fire. The U.N.’s children’s agency said more than 130 children have reportedly been killed.

Lefteris Pitarakis The Associated Press Raed Alsaleh, chairman of the Syrian Civil Defense volunteer rescue group, speaks at a news conference Friday in Istanbul to call attention to an escalation of violence in Idlib province in northwestern Syria.