ANKARA/ISTANBUL: Turkey, Saudi Arabia and some European allies want ground troops deployed in Syria as a Russian-backed government advance nears NATO's southeastern border, Turkey's foreign minister said, but Washington has so far ruled out a major offensive.

Syrian government forces made fresh advances on Tuesday, as did Kurdish militia, both at the expense of rebels whose positions have been collapsing in recent weeks under the Russian-backed onslaught.

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, while Kurdish fighters, regarded by Ankara as hostile insurgents, have extended 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, targeting the Kurdish YPG militia which Ankara says is being backed by Moscow.

"Some countries like us, Saudi Arabia and some other Western European countries have said that a ground operation is necessary," Turkish Foreign Minister Mevlut Cavusoglu told Reuters in an interview.

However, this kind of action could not be left to regional powers alone. "To expect this only from Saudi Arabia, Turkey and Qatar is neither right nor realistic. If such an operation is to take place, it has to be carried out jointly, like the (coalition) air strikes," he said.

Turkish President Tayyip Erdogan said the "brutal operation" by Russian and Syrian forces was aimed at forging a YPG corridor along Turkey's border, something Ankara has long feared would fuel Kurdish separatist ambition on its own soil.

Turkey accused Russia on Monday 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.

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

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.

The UN Syria envoy, Staffan de Mistura, held talks with Syria's foreign minister on Tuesday aimed at securing a cessation of hostilities and said Damascus had a duty to let the world body bring in humanitarian aid.

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, their objective since 2011 with the encouragement of the West, Arab states and Turkey.

SYRIAN MILITARY GAINS

Kurdish forces continued their push eastwards towards Islamic State-held territory northeast of Aleppo.

The Syrian Observatory for Human Rights, a British-based group which monitors the war, said the Kurdish-backed Syria Democratic Forces (SDF) - of which the YPG is a part - took a village near the town of Marea. That is the last major settlement before territory held by the radical militants stretching into Iraq.

The Syrian army also made advances, with state media saying it had taken two villages north of Aleppo near the town of Tal Rifaat, which fell to the SDF on Monday. With the help of Russian air strikes it also advanced from the coastal city of Latakia, fighting to take the town of Kansaba.

With hundreds of thousands trapped in areas the government aims to seize, 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 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."

DOUBTS OVER GROUND TROOPS

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 United States sees the YPG as one of the few effective ground forces fighting Islamic State militants in Syria, and has lent the group military support.

Washington has so far ruled out sending its own ground troops into Syria, apart from small numbers of special forces.

Sunni Arab Gulf states including Saudi Arabia and the United Arab Emirates (UAE) said this month they were ready to send ground forces as part of an international coalition against Islamic State, providing Washington takes the lead.

But Turkey's focus on the YPG means it cannot necessarily count on support from NATO, which, while reluctant to pressure Ankara in public, is working behind closed doors to discourage it from targeting the Kurds and escalating with Russia.