KIEV/BEIRUT (Reuters) – Turkey warned Kurdish militia fighters in northern Syria on Monday they would face the “harshest reaction” if they tried to capture a town near the Turkish border, and accused Russia of a missile attack there that killed at least 14 civilians.

An offensive supported by Russian bombing and Iranian-backed Shi’ite militias has brought the Syrian army to within 15 miles of Turkey’s border. The Kurdish YPG militia has exploited the situation, seizing ground from Syrian rebels to extend its presence along the frontier.

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 laws.

At least 14 were killed in the 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.

Turkish Prime Minister Ahmet Davutoglu said a Russian missile had hit the buildings and that many civilians including children had been killed.

Turkey shelled YPG positions for a third day to try to stop its fighters seizing Azaz, just 5 miles from the border. Ankara fears the Kurdish militia, backed by Russia, are trying to secure the last stretch of around 60 miles along the Syrian border not already under its control.

“We will not allow Azaz to fall,” Davutoglu told reporters on his plane on the way to Ukraine.

YPG fighters would already have taken Azaz and Tal Rifaat further south had it not been for Turkish artillery firing at them over the weekend, he said.

“If they approach again they will see the harshest reaction,” he said.

The standoff has increased the risk of direct confrontation between Russia and NATO member Turkey.

The Syrian civil war, reshaped by Russia’s intervention last September, has gone into an even higher gear since the United Nations sought to revive peace talks. These were suspended earlier this month in Geneva 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.

U.N ENVOY IN DAMASCUS

U.N. Syria envoy Staffan de Mistura made a surprise visit to Damascus on Monday and will hold talks with Syria’s foreign minister on Tuesday, a Syrian government official told Reuters. The talks will include discussion of the resumption of the Geneva peace talks later this month, the official said.

In a further clouding of the Munich deal, Assad said on Monday that any ceasefire did not mean each side had to stop using weapons, and that nobody was capable of securing the conditions for one within a week.

At a news conference in Kiev, Davutoglu doubted Russia’s commitment to any such deal to cease hostilities, pointing to comments from Moscow that it would continue its air strikes regardless.

Russia, Davutoglu said, had a clear objective: “They want to have just two options in front of the international community: Daesh or Assad,” he said, using an Arabic acronym for Islamic State.

Turkey is enraged by the expansion of Kurdish influence in northern Syria, fearing it will encourage separatist ambitions among its own Kurds. It considers the YPG a terrorist group.

Davutoglu said Turkey would make the Menagh air base north of the city of Aleppo “unusable” if the YPG, which seized it over the weekend from Syrian insurgents, did not withdraw.

He warned the YPG not to move east of the Afrin region or west of the Euphrates River, long a “red line” for Ankara.

Turkish Defense Minister Ismet Yilmaz said Ankara was not considering sending troops to Syria, according to the state-run Anadolu Agency.

Syria’s rebels, some backed by the United States, Turkey and their allies, say the YPG is fighting with the Syrian military and its backers, including Russia, against them in the five-year-old civil war. The YPG denies this.

South of Azaz, the Kurdish-backed Syria Democratic Forces (SDF) alliance, of which the YPG is a member, took around 70 percent of the town of Tal Rifaat, according to the Syrian Observatory, which monitors the war.

HOSPITALS HIT

Tens of thousands have fled to Azaz from towns and villages where there is heavy fighting between the Syrian army and militias.

“We have been moving scores of screaming children from the hospital,” said medic Juma Rahal, following the missile strikes. At least two children were killed and ambulances ferried scores of injured people to Turkey for treatment, he said.

French charity Médecins Sans Frontières (MSF) meanwhile said seven people were killed and at least eight staff were missing after missiles hit a hospital in the province of Idlib, west of Aleppo, in a separate incident.

“The author of the strike is clearly…either the government or Russia,” MSF president Mego Terzian said.

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 defense forces. This contradicts our ideology,” she said in Geneva.

Syria’s ambassador to Russia meanwhile said U.S. war planes were responsible.

“Concerning the hospital which was destroyed, in actual fact it was destroyed by the American Air Force. The Russian Air Force has nothing to do it with,” Ambassador Riad Haddad told Rossiya 24 television.

Ankara views the YPG as an extension of the Kurdistan Workers Party (PKK), which has fought a 31-year-old insurgency for autonomy in southeast Turkey. But Washington, which does not see the YPG as terrorists, supports the group in the fight against Islamic State in Syria.

That has left Turkey dangerously exposed, unable to count on the support of its NATO allies as it campaigns against the YPG, but also threatened by Islamic State fighters, as well as Syrian government forces and their backers including Russia.

Turkish financial markets were weaker on Monday on fears about the situation on the border, with the lira underperforming emerging markets currencies, its dollar bonds selling off heavily, and its default insurance costs rising.

(Additional reporting by Lisa Barrington in Beirut, Humeyra Pamuk in Istanbul, Tulay Karadeniz, Orhan Coskun and Ece Toksabay in Ankara, Stephanie Nebehay in Geneva, Jack Stubbs in Moscow; Writing by Nick Tattersall; Editing by David Dolan, Giles Elgood and Pravin Char)