More than one million children have fled Ukraine to neighboring countries since the start of the Russian invasion nearly two weeks ago, according to the executive director of the United Nations Children's Fund (UNICEF) on Wednesday. 

At least 37 children have been killed and 50 were injured in the weeks since Russian President Vladimir Putin ordered the invasion of Ukraine on Feb. 24., UNICEF's Catherine Russell said in a statement.

Russell said she was "horrified" by the reported attack on a maternity hospital in Mariupol, Ukraine, on Wednesday that wounded more than a dozen people, including women near childbirth, Ukrainian officials said, according to the Associated Press. 

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Ukrainian family at train station to escape war

On March 1 2022, a family travels with small children to the Lviv-Holovnyi railway station in Lviv, Ukraine as the Russia-Ukraine war continues. (UNICEF/Viktor Moskaliuk)

"I am horrified by the reported attack today on a maternity hospital in Mariupol, Ukraine – an attack which reportedly left young children and women in labor buried beneath the rubble of destroyed buildings," Russell said. 

Catherine Russell C front, head of the UN Children's Fund, speaks during the Security Council meeting on Ukraine at the UN headquarters in New York, March 7, 2022. ((Photo by Xie E/Xinhua via Getty Images))

"This attack, if confirmed, underscores the horrific toll this war is exacting on Ukraine’s children and families," she added. "In less than two weeks, at least 37 children have been killed and 50 injured, while more than 1 million children have fled Ukraine to neighboring countries."

Attacks from Russian forces on residential buildings and hospitals have forced families and medical patients, including expectant mothers, to take shelter underground and receive medical services in makeshift shelters.

Ukrainian President Volodymyr Zelenskyy shared video of what he described as the "direct strike of Russian troops" on the maternity hospital in Mariupol. He once again demanded the skies be closed over Ukraine. 

"People, children are under the wreckage. Atrocity!" Zelenskyy tweeted. "How much longer will the world be an accomplice ignoring terror? Close the sky right now! Stop the killings! You have power but you seem to be losing humanity."

More than 2 million people have fled Ukraine since Putin approved a "special military operation" in the country last month, according to a tracker from the United Nations refugee agency. 

Reuters reported that those who fled are mostly women and children because the Kyiv government has ordered able-bodied men to stay in the country and fight.

But many of the children fleeing are either unaccompanied or have been separated from their parents or family members, UNICEF says. The agency warned that children without parental care are at a "heightened risk of violence, abuse, and exploitation."

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"When these children are moved across borders, the risks are multiplied. The risk of trafficking also soars in emergencies," UNICEF added. 

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Russell noted that the children of Ukraine "desperately need peace."

Fox News' Audrey Conklin contributed to this report.