By Ercan Gurses and Suleiman Al-Khalidi
Turkey has accused Russia of an "obvious war crime" after missile attacks in northern Syria killed scores of people, and warned Kurdish militia fighters there they would face the "harshest reaction" if they tried to capture a town near the Turkish border.
An offensive supported by Russian bombing and Iranian-backed Shi'ite militias has brought the Syrian army to within 25km 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.
Almost 50 civilians were killed when missiles hit at least five medical facilities and two schools in rebel-held areas of Syria on Monday (local time), 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, when missiles hit a children's hospital and a school sheltering refugees, a medic and two residents said.
Missiles also hit a hospital in the town of Marat Numan in the province of Idlib, south of Aleppo.
Turkish Prime Minister Ahmet Davutoglu said a Russian missile had hit the buildings and that many civilians including children had been killed.
Turkey's foreign ministry accused Russia of carrying out an "obvious war crime".
But Russian Health Minister Veronika Skvortsova said Russian air strikes were targeting Islamic State infrastructure and she had no reason to believe that Russian planes had bombed civilian sites in Idlib.
"We are confident that [there is] no way could it be done by our defence forces. This contradicts our ideology," she said in Geneva.
Syria's ambassador to Russia said US war planes were responsible.
White House national security adviser Susan Rice on Monday condemned in the "strongest terms" the intensified bombing of northern Syria, adding that it ran counter to commitments to reduce hostilities made by major powers last week in Munich.
The Syrian civil war, reshaped by Russia's intervention last September, has gone into an even higher gear since the UN sought to revive peace talks.
The talks in Geneva were suspended earlier this month before they got off the ground.
World powers agreed in Munich on Friday to a cessation of hostilities in Syria to allow humanitarian aid to be delivered, but the deal does not take effect until the end of this week and was not signed by any warring parties.