The US State Department has said Syria's air strike on a hospital in Aleppo was "reprehensible," and has called on Russia to use its influence to pressure Syrian President Bashar al-Assad's government to stop the attacks.
State Department spokesman John Kirby said the US was still learning more about the attack on a hospital supported by the Médecins Sans Frontières (MSF) medical charity but that there appeared to be several dozen casualties.
"Once again we call on the regime to cease these absolutely senseless attacks, which are of course violations of the cessation of hostilities," Mr Kirby said, calling the bombing "reprehensible."
He said the attack bore the hallmarks of attacks that the Syrian government has carried out on medical facilities and first responders in the past.
Mr Kirby said the Syrian cessation of hostilities was "very much in peril" because of the ongoing violations.
The Doctors Without Borders organisation says at least 14 patients and staff, including three doctors, were killed in the strike on the Al Quds hospital.
A statement from the organisation said the hospital was well known locally to all parties in the conflict.
Muskilda Zancada, head of MSF mission in Syria said: "MSF categorically condemns this outrageous targeting and destruction of yet another medical facility in Syria.
"This devastating attack has destroyed a vital hospital in Aleppo, and the main referral centre for paediatric care in the area."
UN special envoy for Syria Staffan de Mistura has said he did not believe the targeting of the hospital was a mistake, although he did not expand on who he believed carried out the strike.
Earlier, the US said the attacks bore the hallmarks of previous strikes carried out by Syrian government forces.
Mr de Mistura also called on the United States and Russia to intervene to salvage a “barely alive” ceasefire deal.
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 British-based 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.
"I could not in any way express how high the stakes are for the next hours and days."
Talks in Geneva aim to end a war that has created the world's worst refugee crisis, allowed for the rise of the so-called Islamic State group 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 Geneva talks, Mr de Mistura said he aimed to resume them in May, but gave no date.
A Syrian military source said government planes had not been in areas where air raids were reported. Syria's army denied reports that the Syrian air force targetted 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.
