Turkey is asking allies including the United States to take part in a joint ground operation in Syria, as a Moscow-backed government advance nears its borders, raising the possibility of direct confrontation between the NATO member and Russia.
A large-scale joint ground operation is still unlikely: Washington has ruled out a major offensive. But the request shows how swiftly a Russian-backed advance in recent weeks has transformed a conflict that has drawn in most regional and global powers.
The offensive, supported by Iranian-backed Shi'ite militias as well as Russian air strikes, has brought the Syrian army to within 25 km (15 miles) of Turkey's frontier. Kurdish fighters regarded by Turkey as hostile insurgents have also exploited the collapse of positions held by other rebel groups to seize ground and extend their presence along the border.
The advances have increased the risk of a military confrontation between Russia and Turkey. Turkish artillery returned fire into Syria for a fourth straight day on Tuesday, military sources said, targeting the Kurdish YPG militia which Ankara says is being backed by Moscow.
"We want a ground operation. If there is a consensus, Turkey will take part. Without a ground operation it is impossible to stop this war," a Turkish official told reporters at a briefing in Istanbul.
"Turkey is not going to have a unilateral ground operation ... We are discussing this with allies," the official said, declining to be named in order to speak more freely.
Russian air support for the Syrian government offensive has transformed the balance of power in the 5-year-old war in the past three weeks.
Damascus says its objectives are to recapture Aleppo, Syria's biggest city before the war, and seal off the border with Turkey that has served as the main supply route into rebel held territory for years.
Those would be the government's biggest victories of the war so far and probably end rebel hopes of overthrowing President Bashar al-Assad by force, the objective they have sought since 2011 with the encouragement of the West, Arab states and Turkey.
Turkey's focus on the threat from the YPG means it cannot necessarily count on such support from NATO, which, while reluctant to pressure Ankara in public, is working behind closed doors to discourage it from targeting the Kurds and from an escalation with Russia.
"The Kurds are part of the conflict in Syria, but also in Iraq, and therefore they should also be part of the solution," NATO Secretary General Jens Stoltenberg said at the Munich Security Conference over the weekend.
German Foreign Minister Frank-Walter Steinmeier urged both Russia and Turkey to calm hostilities.
"It is my clear expectation that Moscow and Ankara adhere, in their military and political approach, to the commitments made in Munich, and that we see a measurable reduction in military activities even before final agreement on a ceasefire," he said in a statement on Tuesday.