Turkey is asking allies including the US 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 25km 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.
Turkish artillery returned fire into Syria for a fourth straight day on Tuesday, the defence minister 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 on Tuesday.
"Turkey is not going to have a unilateral ground operation ... We are discussing this with allies."
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 capture a town near the Turkish border.
World powers meeting in Munich last week agreed to a pause in the fighting, but that is not set to begin until the end of this week and was not signed by the warring Syrian parties.
Meanwhile, 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.
Turkey and others accuse Moscow of deliberately firing on civilian targets such as hospitals to force residents to flee and depopulate territory.
Almost 50 civilians were killed when missiles hit at least five medical facilities and two schools in rebel-held areas of Syria on Monday, according to the United Nations, which called the attacks a blatant violation of international law.
At least 14 were killed in the northern town of Azaz, the last rebel stronghold before the border with Turkey north of Aleppo. Prime Minister Ahmet Davutoglu said a Russian missile was responsible and vowed that Turkey would not let Azaz fall into YPG hands.
Russia's foreign ministry said Turkey was using Azaz as a supply route for Islamic State and "other terrorist groups", while the Kremlin strongly rejected Turkish accusations it had committed a war crime after the missile strikes.
"We categorically do not accept such statements, the more so as every time those making these statements are unable to prove their unfounded accusations in any way," President Vladimir Putin's spokesman Dmitry Peskov told reporters.
"Our relations (with Turkey) are in a deep crisis. Russia regrets this. We are not the initiators of this."
The advances by the YPG risk creating friction between Turkey and its allies, including the United States.
Ankara sees the Syrian Kurdish militia as an extension of the Kurdistan Workers Party (PKK) which has fought a three-decade insurgency for Kurdish autonomy in Turkey's southeast.
But the US sees the YPG as one of few effective ground forces fighting Islamic State militants in Syria, and has lent the group military support.
German Foreign Minister Frank-Walter Steinmeier urged both Russia and Turkey to calm hostilities.
