Three hospitals have been hit in air strikes in northern Syria, medics and witnesses say, killing a number of people.
Medecins Sans Frontieres said seven people died in an attack in Maarat al-Numan. Activists said another hospital in the town was also hit.
Another strike in Azaz near the Turkish border killed 10, reports said.
MSF blamed pro-Syrian government forces for the raid in on its hospital; Turkey blamed Russia for the Azaz strike.
They come days after Russia and other world powers agreed to a limited cessation of hostilities in Syria.
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.
It has not been confirmed who carried out the latest attacks.
However, Mego Terzian, president of MSF France, said the Maarat al-Numan strikes were carried out by forces “loyal to President Bashar al-Assad”.
He told Reuters: “There were at least seven deaths among the personnel and the patients, and at least eight MSF personnel have disappeared, and we don’t know if they are alive.”
Turkish Prime Minister Ahmet Davutoglu said a Russian ballistic missile had hit buildings in Azaz, with children among the dead.
Associated Press reported that five people had died at a children’s hospital.
One medic, Juma Rahal, told the Reuters news agency: “We have been moving scores of screaming children from the hospital.”
Several more people were killed in an air raid on a nearby village, AP reported.
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.
Azaz has seen an influx of thousands of people fleeing advances by the Russian-backed Syrian army in Aleppo province.
The Kurdish YPG militia, which has been making advances in the area, has also targeted Azaz.
Turkey has shelled Kurdish positions since the weekend and on Monday promised the “harshest reaction” if the forces tried to take 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.
MSF said four rockets had hit the hospital in Maarat al-Numan, a rebel-held town about 30km (20 miles) south of the city of Idlib, within minutes of each other on Monday morning.
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.
MSF’s head of mission in Syria, Massimiliano Rebaudengo, said the strikes appeared to be a “deliberate attack on a health structure”, warning the attack left tens of thousands without medical care.
Another hospital in Maarat al-Numan was also hit, opposition group the Local Co-ordination Committees said, killing three people.
Syria’s health facilities targeted
The World Health Organisation (WHO) says 2015 “saw the most blatant attacks on health facilities in Syria”. Between June and August, there were approximately 70 aerial attacks alone
Between the start of the uprising in March 2011 and November 2015, Physicians for Human Rights (PHR) documented 336 attacks on at least 240 medical facilities and the deaths of 697 medical personnel
Ninety percent of the attacks were carried out by the Syrian government and its allies, according to PHR
The UN says an estimated 58% of public hospitals and 49% of primary health centres are either only partially functional or have closed, limiting healthcare workers’ ability to cope with the more than 25,000 trauma cases per month
More than 40% of the population in Syria lack access to basic health services, and the shortages of specialised medical staff, ambulances, equipment and medical supplies have led to an increased number of preventable deaths
MSF, which operates medical facilities inside Syria and supports directly more than 150 others, said last week that attacks were further depleting Syria’s already exhausted healthcare system and preventing more people from accessing desperately needed medical care.
Since the start of this year alone, 14 health facilities in Syria have been hit, which MSF said confirmed that hospitals and clinics were no longer places where patients could recover in safety.
Last Thursday in Munich, 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.