Seven people have died in an air strike on a Medecins Sans Frontieres hospital in northern Syria, the organisation said, calling it a "deliberate" attack.
MSF blamed Syria's government or Russia for the raid in Maarat al-Numan. There has been no independent confirmation.
A further eight people are missing from the strikes, the medical charity said.
Separate strikes on a hospital in the same town, as well as on two more hospitals and a school in Azaz, killed at least 12 people, reports said.
Attacks on medical facilities are forbidden in conflict zones under international humanitarian law.
The raids in northern Syria come days after Russia and other world powers agreed to a limited cessation of hostilities, to begin later this week.
Almost five years of civil war in Syria have led to the deaths of more than 250,000 people. More than 11 million people have been displaced.
MSF anger
MSF said its hospital in Maarat al-Numan was hit by four missiles within minutes of each other.
This "leads us to believe that... it wasn't an accidental attack, that it was deliberate," said Sam Taylor, the spokesman for MSF operations in Syria.
Mego Terzian, president of MSF France, told Reuters "either the [Syrian] government or Russia" was "clearly" responsible.
Monday's attack in Idlib province leaves tens of thousands without medical care, MSF warned.
The Syrian Observatory for Human Rights, a UK-based group which relies on a network of sources on the ground, said nine people were killed, including a child. The raid also left dozens of others wounded, it added.
Another hospital in Maarat al-Numan was also hit, killing three people, said another opposition group, the Local Co-ordination Committees.
The strikes follow a pattern of systematic attacks on healthcare facilities in Syria, says the BBC's Mark Lowen in neighbouring Turkey.
Hospital and school struck in Azaz
In Azaz, near the Turkish border, at least 12 people were killed in an attack on a children's hospital and school, according to activists and witnesses.
One medic, Juma Rahal, told the Reuters news agency: "We have been moving scores of screaming children from the hospital".
There has been no independent confirmation over who was behind either attack in Azaz or Maarat al-Numan.
Unicef said four medical facilities across Syria had been struck: two in Idlib province, two in Azaz. Two schools in Azaz were also hit, the organisation said, reportedly killing six children.
Turkish Prime Minister Ahmet Davutoglu said a Russian ballistic missile had hit buildings in Azaz, with children among the dead.
Russia has been carrying out air strikes in Syria since September in support of President Assad and against what it terms "terrorists".
The strategic supply corridor - Selin Girit, BBC Turkish
Azaz and Tal Rifat are on the corridor stretching from the Turkish border to the city of Aleppo. This is the supply route, the lifeline for the anti-Assad rebels in the area, as it serves as a land bridge to Turkey.
The route faces threats from various sides. To the east, the so-called Islamic State group; to the west the Syrian Kurds; and to the south the Assad forces.
Tal Rifat is 20km (12 miles) from the Turkish border, Azaz just 7km. Halfway between the two is the Mennagh air base, now captured by Syrian Kurds.
Turkey says the Kurds have to retreat - otherwise its shelling of their positions will continue.
A Kurdish capture of Azaz and Tal Rifat - and the fall of the supply corridor - could change things dramatically. Turkey could indeed become directly involved in Syria's war.
Turkish warning
For a third day, Turkey has shelled Kurdish forces in northern Syria trying to take Azaz.
The Kurdish YPG militia has been making advances at the expense of other rebels stretched by a government offensive.
Ahmet Davutoglu promised the "harshest reaction" if the YPG tried to capture Azaz
Turkey views the YPG militia in Syria as allied to the outlawed PKK, which has carried out a decades-long campaign for Kurdish autonomy within Turkey.
Syria said the Turkish shelling was a violation of its sovereignty and has called on the UN Security Council to act.
Last Thursday, world leaders pledged to work towards a cessation of hostilities in Syria within a week,
But Russia argues that the "cessation" does not apply to its air strikes, which have tilted the balance of the war in favour of the Syrian government.
( Source )