The Médecins Sans Frontières medical charity has called for an independent investigation into air strikes that killed 25 people at an MSF-backed hospital in north Syria early this week.
MSF, also known as Doctors Without Borders, said the attack was probably carried out by Syrian and Russian forces as part of an offensive.
Russian-backed Syrian government troops have been pushing toward the rebel stronghold of Aleppo.
In all at least 50 civilians were killed when missiles hit five medical centres and two schools in rebel-held towns on Monday, the United Nations and residents said.
An MSF-supported hospital in Marat Numan in Idlib province, west of Aleppo, was among those struck repeatedly, killing nine medical personnel and 16 patients, it said. Ten others were wounded in the attack that destroyed the 30-bed facility.
"According to accounts from medical staff onsite, four missiles struck the hospital in an attack lasting about two minutes. Forty minutes later, after rescuers arrived, the site was bombed again," said Dr Joanne Liu, international president of the medical charity.
"This attack can only be considered deliberate. It was probably carried out by the Syrian government-led coalition that is predominantly active in the region," she told a news briefing.
Accounts from surviving hospital staff led MSF to believe that the government-led coalition had carried out the attack.
"We say a probability because we don't have more facts than the accounts from our staff," Dr Liu said, noting that it took time to collect forensic evidence. "The only thing predominantly in the region is the Syrian government-led coalition."
"We would like the facts to be established, we are open for other mechanisms for an independent investigation," she said.
MSF said it had not provided the hospital's GPS coordinates to Syrian or Russian authorities, at the request of local staff.
"In Syria the problem faced by medical staff is if you give GPS (coordinates), you indicate where you are, they think the chance to be targeted is higher," Isabelle Defourny, director of operations for MSF France, told the briefing.
"It is known that providing humanitarian assistance inside opposition-controlled areas is something which is criminialised by the Syrian government," she said.
UN chief calls for peaceful settlement
UN Secretary-General Ban Ki-moon is urging Syria's warring sides and the countries backing them to turn away from the "bankrupt logic of a military victory" and work to find a peace settlement, according to a report to the Security Council.
Mr Ban warned that the recent upsurge in fighting in northern Aleppo province risked derailing efforts by UN envoy Staffan de Mistura to convene a new round of peace talks on 25 February.
"Rarely is the international community and this council presented with as stark a choice as the one it now has before it," Mr Ban said in the report obtained by AFP.
On the one hand, world powers can press ahead with agreements on humanitarian aid, work to de-escalate violence, fight terrorism and resume negotiations, he said.
"On the other, the Syrian parties and their supporters can continue to pursue the bankrupt logic of a military victory", he added.
That option has "already led to the deaths of over 250,000 Syrians, the greatest humanitarian crisis of our time, and the creation of safe havens for terrorist organizations" such as the so-called Islamic State group and Al-Nusra Front, said Mr Ban.
The warning came as Turkey pushed for a ground operation in Syria with its allies and Saudi Arabia said it was ready to send forces as part of the US-led coalition.
The report was the first by the UN chief since the council adopted a resolution in December that laid out a roadmap to end the nearly five-year war in Syria.
At their meeting in Munich last week, the 17-nation group backing Syria's peace process agreed to work for a ceasefire, the lifting of starvation sieges and the resumption of talks.
Mr Ban renewed his call to the council to refer the situation in Syria to the International Criminal Court to ensure those responsible for "appalling crimes" face justice.
The council is expected to hear a report from Mr de Mistura on Wednesday.
At least 38 reportedly killed in US-led strikes in Syria
At least 38 people have reportedly been killed in US-led airstrikes in Syria, according to a British-based monitoring group.
The Syrian Observatory for Human Rights said at least 38 civilians were killed in air strikes carried out by a US-led coalition in Hasaka province in northeast Syria in the past two days.
The United States and its allies are carrying out air raids in the area against so-called Islamic State militants, which controls some parts of Hasaka province but has lost ground in recent months to a US-backed alliance that includes the powerful Kurdish YPG militia.
Hasaka neighbours mostly IS-held Deir al-Zor province and Raqqa, the group's de-facto capital in Syria.
The death toll published by the Observatory, which tracks the war using a network of contacts on the ground, included at least 15 people killed when strikes hit a bakery in the city of al-Shadadi near the border with Iraq on Tuesday.
