Turkey called on Tuesday allies including the United States to take part in a joint ground operation in Syria, especially with Moscow-backed government advance nears its borders, which raises the possibility of direct confrontation between the NATO member and Russia.

“We want a ground operation. If there is a consensus, Turkey will take part,” Reuters cited a Turkish official, who refused to be named in order to speak more freely. “Without a ground operation it is impossible to stop this war.” There was no immediate U.S. response.

A large-scale joint ground operation is still improbable given that 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.

A Syrian government offensive in recent weeks, supported by Iranian-backed Shiite militias and Russian airstrikes, has brought President Bashar Assad’s forces within 25 km (15 miles) of Turkey’s frontier.

Kurdish fighters regarded by Turkey as hostile insurgents have also exploited the chaos by seizing positions held by other Syrian opposition fighters and extending their presence along the border.

The advances have amplified the risk of a military confrontation between Russia and Turkey. Turkish artillery returned fire into Syria for a fourth straight day on Tuesday, the defense minister said, targeting the Kurdish YPG militia which Ankara says is being supported by Moscow.

“Turkey is not going to have a unilateral ground operation … We are discussing this with allies,” the official said.

“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.

The NATO member has already has been opening up its air bases to the U.S. and other members of the coalition fighting ISIS group in Syria.

Turkey on Monday accused Russia of an “obvious war crime” after missile attacks in northern Syria killed scores of people, and warned the YPG it would face the “harshest reaction” if it tried to seize a town near the Turkish border.

Russian air support for the Syrian government offensive has altered the balance of power in the 5-year-old war in the past three weeks.

World powers meeting in Munich last week agreed to a cessation of hostilities, that is not set to begin until the end of this week and was not signed by the warring Syrian parties.