By Lisa Barrington and Stephanie Nebehay
Air strikes have destroyed a hospital and killed dozens of people in rebel-held areas of Aleppo, including children and doctors, in an attack that a US official says appears to be solely the work of the Syrian government.
The city of Aleppo is at the centre of a military escalation that has undermined peace talks in Geneva aimed at ending the five-year-old war.
UN envoy Staffan de Mistura on Thursday (local time) appealed to the presidents of the US 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 jeopardised the aid lifeline that supplies millions of Syrians, said Jan Egeland, chairman of the UN humanitarian task force.
The US State Department said Syria's air strike on the hospital in Aleppo was "reprehensible," and it called on Russia to use its influence to pressure Syrian President Bashar al-Assad's government to stop the attacks.
US 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 world's 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.
Syria's army denied reports that the Syrian air force targeted the hospital.
The Russian defence ministry, whose air strikes have swung the war in favour of President Bashar al-Assad, also denied its planes were responsible.
The British-based Observatory said 31 people were killed as a result of air strikes on several areas of opposition-held Aleppo on Thursday.
In addition, it said at least 27 people were killed in the air strike on the hospital late on Wednesday.
Rescue workers put the toll higher.
In government areas, rebel mortar shelling killed at least 14 people, the Observatory and Syria's state news agency SANA reported.
The bombed al-Quds hospital was supported by international medical charity Medecins Sans Frontieres (MSF), which said it was destroyed after being hit by a direct air strike that killed at least three doctors.
"This devastating attack has destroyed a vital hospital in Aleppo, and the main referral centre for paediatric care in the area," said Musil Sanada, MSF head of mission, Syria. "Where is the outrage among those with the power and obligation to stop this carnage?"