BEIRUT -- An airstrike destroyed a makeshift clinic supported by an international aid group in northern Syria on Monday, killing and wounding several people, activists and aid officials said.

The attack was part of an apparent series of strikes on medical facilities and schools near the front lines of Syria's bloody civil war, and have killed a total of about 50 civilians, U.N. officials said Monday.

Mirella Hodeib of Doctors Without Borders -- also known by its French acronym MSF -- said one airstrike destroyed the MSF-supported structure in the northwestern town of Maaret al-Numan in Idlib province. She had no immediate word on casualties or the circumstances of the strike.

The Britain-based Syrian Observatory for Human Rights said Russian warplanes targeted the makeshift hospital, destroying it and killing nine people.

Get Breaking News Delivered to Your Inbox

The Observatory, which tracks the casualties in Syria's five-year civil war, said dozens were also wounded in the airstrike.

U.N. Secretary-General Ban Ki-moon said close to 50 civilians have been killed and many more wounded in missile attacks on at least five medical facilities and two schools in northern Syria.

U.N. deputy spokesman Farhan Haq said Monday that victims of the attacks included children.

He quotes the secretary-general as calling the attacks "blatant violations of international laws" that "are further degrading an already devastated health care system and preventing access to education in Syria."

Haq quoted Ban as saying the attacks "cast a shadow on commitments" made by nations seeking to end the Syrian conflict at a conference in Munich on Feb. 11, which included a cessation of hostilities within a week and an end to attacks on civilians.

The head of the U.N. children's agency said he is "appalled" at reports they've received of attacks on four medical facilities in Syria - two supported by UNICEF.

UNICEF Executive Director Anthony Lake said Monday that one facility was a child and maternal hospital where children were reportedly killed and scores evacuated. He said two strikes took place in the northern city of Azaz and two in the Idlib province.

He said there are also reports that two schools were attacked at Azaz, reportedly killing six children. Lake says a third of hospitals and a quarter of schools in Syria are no longer functioning. Aside from diplomatic considerations and obligations under international humanitarian law, Lake said, "let us remember that these victims are children. Children."

In a statement, the U.S. State Department said: "That the Assad regime and its supporters would continue these attacks, without cause and without sufficient regard for international obligations to safeguard innocent lives, flies in the face of the unanimous calls by the ISSG, including in Munich, to avoid attacks on civilians and casts doubt on Russia's willingness and/or ability to help bring to a stop the continued brutality of the Assad regime against its own people."

Syrian troops have been on the offensive in northern Syria under the cover of Russian airstrikes over the past week. The ground offensive has been focused on the northern province of Aleppo while Monday's airstrike struck the clinic in the nearby Idlib province.

"The entire building has collapsed on the ground," said opposition activist Yahya al-Sobeih, speaking by telephone from Maaret al-Numan. He added that five people were killed near the clinic and "all members of the medical team inside are believed to be dead."

He added that paramedics and workers are now working on removing the rubble. Al-Sobeih said the four-story building, once a cement company but which had served as a makeshift clinic during the five-year civil war, was hit with four missiles.

An aid official said at least one patient died and nine Syrian staffers were missing. The official, who was not authorized to talk to reporters and spoke on condition of anonymity, did not provide more details. Casualty figures are often sketchy and conflicting, and cannot be independently verified because of the inaccessibility of the conflict zones.

Meanwhile in Brussels, European Union officials on Monday called on Turkey to halt its military action in Syria after Turkish forces shelled positions held by a U.S.-backed Kurdish militia over the weekend.

The EU's foreign policy chief, Federica Mogherini, said that "only a few days ago, all of us including Turkey, sitting around the table decided steps to de-escalate and have a cessation of hostilities."

She said more fighting "is obviously not what we expect."

Dutch Foreign Minister Bert Koenders, whose country holds the EU's rotating presidency, said "we have the plan for a cessation of hostilities and I think everybody has to abide by that."

Syria's main Kurdish faction, the People's Protection Units, has been most effective in combating the Islamic State of Iraq and Syria (ISIS), but Turkey appears uneasy over the group's recent gains.