The Kremlin has strongly rejected accusations by Turkey that Russia committed an "obvious war crime" in Syria after missile attacks killed scores of people on Monday, hitting several medical facilities and schools.

"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 during a conference call on Tuesday.

Turkey blamed Russia for Monday's attacks, which saw almost 50 civilians killed when missiles hit at least five medical facilities — including an Médecins Sans Frontières (MSF)-supported hospital — and two schools in rebel-held areas of Syria. Both the United Nations and France have also condemned the attacks, with the UN calling them a blatant violation of international law, while France echoed Turkey in saying they constituted war crimes.

At least 14 people died on Monday in the northern Syrian town of Azaz, the last rebel stronghold before the border with Turkey, when missiles hit a children's hospital and a school sheltering refugees.

This comes as Turkey is asking its coalition partners, including the US, to take part in a joint ground operation in Syria to try to end the five-year-old civil war, a Turkish official said on Tuesday.

The official said it was now impossible to stop the war without such an operation, but that Ankara would not launch such an offensive on its own.

"Turkey is not going to have a unilateral ground operation. We are asking coalition partners that there should be a ground operation. We are discussing this with allies," the official told reporters at a briefing in Istanbul.

"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."

Turkish artillery has also been returning fire "in kind" into Syria, military sources said on Tuesday, marking its fourth straight day of shelling across the border.

On Monday, Turkey warned Kurdish militia fighters in northern Syria they would face the "harshest reaction" if they tried to capture a town near the border. An offensive by the Syrian army, supported by Russian bombing and Iranian-backed Shia militias, has brought it to within 15 miles of Turkey's frontier.

The Kurdish YPG militia, which Turkey regards as a hostile insurgent force, has exploited the situation, seizing ground from Syrian rebels to extend its presence along the border.

A ceasefire planned for later this week also looks unlikely to go ahead, with Syrian President Bashar al-Assad saying on Monday that any such action did not mean each side had to stop using weapons, and adding that nobody was capable of securing the conditions for one within a week.

"A ceasefire means in the first place halting the terrorists from strengthening their positions. Movement of weapons, equipment or terrorists, or fortification of positions, will not be allowed," Assad said in Damascus in televised comments.