The head of the United Nations Children's Fund (UNICEF) said Wednesday she is "horrified" by an attack on a maternity hospital in Mariupol, Ukraine which has reportedly left young children and women in labor buried beneath the rubble.

"We do not yet know the number of casualties but fear the worst," said UNICEF Executive Director Catherine Russell.

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

Russell said that in less than two weeks since Russian armed forces invaded Ukraine, at least 37 children have been killed and 50 injured while more than 1 million children have fled Ukraine to neighboring countries.

"Attacks against civilians and civilian infrastructure – including hospitals, water and sanitation systems and schools – are unconscionable and must stop immediately," she said.

"I am horrified by the reported attack…in Mariupol," said Russell. "The children of Ukraine desperately need peace."

She said UNICEF renews its call for an immediate cease-fire and urges all parties to respect their obligations under international humanitarian law to protect children from harm and to ensure that humanitarian actors can safely and quickly reach children in need."

World Health Organization (WHO) Director-General Tedros Adhanom Ghebreyesus said before the attack was reported that "so far, the WHO has verified 18 attacks on health facilities, health workers and ambulances," in which there were 10 deaths and 16 injuries.

The day before, the WHO’s Regional Director for Europe, Hans Kluge, told journalists at a press conference that the "WHO strongly condemns these attacks on health care services," after reporting attacks on health services and workers./aa