Syria - Air strikes destroyed a hospital and killed dozens of people in rebel-held areas of Aleppo, including children and doctors, in an attack that a U.S. official said appeared to be solely the work of the Syrian government.
The city of Aleppo is at the center of a military escalation that has undermined peace talks in Geneva aimed at ending the five-year-old war.
U.N. envoy Staffan de Mistura appealed to the presidents of the United States and Russia to intervene to salvage a ceasefire that was "barely alive". The truce is intended to allow an opportunity for peace talks and delivery of humanitarian relief.
Six days of air strikes and rebel shelling in Aleppo, which is split between government and rebel forces, have killed 200 people, two-thirds of them on the opposition side, the Syrian Observatory for Human Rights says.
The "catastrophic deterioration" in Aleppo over the last 24 to 48 hours has jeopardized the aid lifeline that supplies millions of Syrians, said Jan Egeland, chairman of the U.N. humanitarian task force. "I could not in any way express how high the stakes are for the next hours and days."
The U.S. State Department said Syrias air strike on the hospital in Aleppo was "reprehensible," and it called on Russia to use its influence to pressure Syrian President Bashar al-Assads government to stop the attacks.
"Once again we call on the regime to cease these absolutely senseless attacks, which are of course violations of the cessation of hostilities," State Department spokesman John Kirby said.
A State Department official said there were indications that the bombing of the hospital was conducted solely by the Syrian government.
U.S. Secretary of State John Kerry said Russia had an urgent responsibility to press the government of its ally Assad to stop attacking civilians and observe the ceasefire.
The Geneva talks aim to end a war that has created the worlds worst refugee crisis, allowed for the rise of Islamic State and drawn in regional and major powers, but the negotiations have all but failed and a truce to allow them to take place has collapsed.
Winding up the latest round of talks, de Mistura said he aimed to resume them in May, but gave no date.
"Wherever you are, you hear explosions of mortars, shelling and planes flying over," said Valter Gros, who heads the International Committee of the Red Cross Aleppo office.
"There is no neighborhood of the city that hasn’t been hit. People are living on the edge. Everyone here fears for their lives and nobody knows what is coming next," he said.
A Syrian military source said government planes had not been in areas where air raids were reported. Syrias army denied reports that the Syrian air force targeted the hospital.