BEIRUT • The prospect of a wider war loomed over an increasingly chaotic battlefield in northern Syria, as fast-moving clashes pitted United States allies against one another and tensions grew between two major powers, Russia and Turkey.
Taking advantage of fierce Russian air strikes, Kurdish-led forces advanced into shrinking rebel territory in the northern part of Aleppo province, infuriating the insurgents and their principal backer, Turkey, which threatened "a severe response" if the Kurds moved farther.
The situation has pitted a dizzying array of warring parties against one another, illustrating how the enemy of my enemy is, as often as not, my enemy.
Tensions have never been higher between Kurds and Syrian Arab rebels, who now accuse the Kurds of opportunistically attacking them in league with Russia and the Syrian government to advance their goal of seizing territory along the border with Turkey.
Those clashes are fuelling a risky escalation of tensions, especially between Turkey and Russia. And they are pitting two US-backed groups against each other, since both the Kurds and many rebel groups in Aleppo receive American support.
As always, caught in the middle are civilians, with five hospitals and two schools bombed in a single day on Monday, and Turkey and the Syrian insurgents accusing Russia of targeting them deliberately to drive them out of the area.
The United Nations said nearly 50 civilians, including children, died in the bombings in northern Syria's rebel-held Aleppo and Idlib provinces. It did not identify who carried out the air strikes.
UN Secretary-General Ban Ki Moon said the raids violated international law and "cast a shadow" over efforts to end Syria's five-year civil war.
The region around Aleppo has been the target of a major offensive by Syrian government troops, backed by Russian warplanes, which has sent tens of thousands fleeing to the Turkish border.
Two of the hospitals hit on Monday were in Azaz, a critical border town in the fierce battles unfolding in Aleppo. Kurdish militias and Arab allies were advancing closer to Azaz and overran Tal Rifaat, a town on the road to the border from Aleppo.
Turkey, which views the Kurds as its worst enemy in the region and does not want to see more Kurdish-controlled areas along its border, shelled the advancing forces for a third consecutive day. "We will not let Azaz fall," Turkey's Prime Minister Ahmet Davutoglu told reporters on his plane en route to Ukraine, according to the semi-official Anadolu News Agency. He threatened "a severe response" if Kurdish forces advanced on the town.
Russia's Foreign Ministry declared that Turkey, by shelling the Kurds as they battled insurgents, was providing "direct support for international terrorism".
It also insisted that the US-led coalition against the Islamic State in Iraq and Syria was responsible for a hospital bombing in Idlib which killed at least seven people, though US military officials said there were no coalition strikes in the area.
The latest strike came amid days of escalation along the Syria-Turkey border, despite - or, experts say, perhaps because of - the US and Russia having agreed last week to work towards a ceasefire, said to be starting by the end of this week.
NEW YORK TIMES, AGENCE FRANCE-PRESSE