Almost 50 people, including many children, were killed Monday in missile attacks on hospitals and schools in Syria, the United Nations said.
The attacks - at least some of them apparently carried out by Russian forces supporting Syrian President Bashar al-Assad - came four days before an internationally backed truce was slated to start.
UN envoy Staffan de Mistura arrived late Monday in Damascus where he was expected to press the case for the truce and a resumption of peace talks in Geneva.
Al-Assad, however, cast doubts on the prospect of the ceasefire coming into effect on Friday, as announced last week by world powers at an international security conference in Munich.
"Now they say they want a ceasefire within a week, fine, but who is able to draw together all these conditions and requirements within a week? Nobody," al-Assad said.
The push by Western powers for a ceasefire came only in response to losses on the ground by rebel factions they support, he charged in a speech to Syrian lawyers in Damascus.
The UN suspended peace talks in Geneva earlier this month amid opposition fury as a government offensive backed by Russian airstrikes broke rebel lines north of Aleppo and sent tens of thousands of civilians fleeing.
At least five hospitals and two schools were hit in Monday's missile strikes, UN spokesman Farhan Haq said.
The medical charity Doctors Without Borders (MSF) reported earlier that seven people were killed and eight others were presumed dead in a strike on a hospital it supported in Maarat al-Nuaman south of Aleppo.
It denounced the incident as a "deliberate attack" on health structures, but did not specify who was responsible.
The Syrian Observatory for Human Rights, an opposition monitoring group, said it appeared to have been carried out by Russian forces.
Another apparently Russian airstrike hit a second hospital in the town, killing two nurses, the Observatory said.
The Observatory also reported Russian strikes near a school and a hospital in the town of A'zaz, in Aleppo province near the Turkish border, which killed 10 people including three children.
The European Union condemned the attacks, saying they followed "a series of similar assaults against civilian infrastructure happening on an almost daily basis across Syria which are in clear violation of international humanitarian law."
"In the case of Syria, these systematic assaults have already broken the country's health system, which suffers from a severe lack of treatment and medicine," the EU said.
According to medics, at least 10 hospitals in Aleppo province are no longer functional as a result of heavy damage from airstrikes.
The airstrikes on medical facilities in rebel-held territory are all thought to have been carried out by Russian and Syrian forces.
German Chancellor Angela Merkel called for the introduction of a no-fly zone in Syria.
"In the current situation it would be helpful if there were a field to which none of the warring factions can launch attacks by air - a kind of no-fly zone," Merkel told the daily Stuttgarter Zeitung.
Russia, a major ally of Syria, started its strikes in Syria on September 30.
The Russian-backed government offensive earlier this month cut off rebels in Aleppo city from vital supply lines to the Turkish border, putting the city at risk of being besieged.
Kurdish-led forces backed by Russian and government airstrikes meanwhile continued to advance, seizing Tel Rifaat, one of three remaining rebel-held towns between Aleppo and the border, the Observatory said.
Turkey, which has shelled the Kurds repeatedly in recent days, earlier warned them against any attack on A'zaz.
"If they approach A'zaz, they will face the most severe reaction," Prime Minister Ahmet Davutoglu was quoted by the state-run Anadolu news agency as saying while travelling to Kiev.
The Russian Foreign Ministry hit back at Turkey, expressing "very serious concern" at what it described as "the aggressive actions of Turkish authorities regarding their neighbouring state."
Ankara fears the Kurdish forces' links to the banned Kurdistan Workers' Party (PKK), currently fighting Turkish government forces.
Turkey on Monday announced that it was carrying out air defence exercises with the participation of fighter jets from Saudi Arabia, which recently suggested it could take part in ground operations inside Syria against Islamic State militants.
The Syrian Kurds are on good terms with Russia, but they are also the main ally on the ground helping the United States beat back the Islamic State extremist group. This relationship has led to tensions between Turkey and the US.