WASHINGTON - More than 1 million children have fled Ukraine to neighboring countries in the less than two weeks since Russia started its invasion of Ukraine, the head of the United Nations Children's Fund (UNICEF) said on Wednesday.

At least 37 children had been killed and 50 injured, Executive Director Catherine Russell said in a statement.

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Russell said that she was "horrified" by the reported attack on a children's hospital in the Ukrainian city of Mariupol, where officials said a Russian air strike buried patients under rubble despite an agreed ceasefire.

"This attack, if confirmed, underscores the horrific toll this war is exacting on Ukraine’s children and families," Russell said.

The bombing, which Ukrainian President Volodymyr Zelenskiy called an "atrocity," took place despite an agreed ceasefire to enable thousands of civilians trapped in the city to escape.

Mariupol city council said the hospital had been hit several times by an air strike, causing "colossal" destruction. Kremlin spokesman Dmitry Peskov said: "Russian forces do not fire on civilian targets."

More than 2 million people have fled Ukraine since Russian President Vladimir Putin ordered the invasion on Feb. 24. Moscow calls its action a "special military operation" to disarm its neighbor and dislodge leaders it calls "neo-Nazis."

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Most of those who fled are women and children, as able-bodied men have been ordered by the Kyiv government to stay home to fight.

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The war has swiftly cast Russia into economic isolation as well as drawing almost universal international condemnation. The United States on Tuesday banned imports of Russian oil, while Western companies are rapidly pulling out of the Russian market.

(Reporting by Humeyra Pamuk; editing by Grant McCool)

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