One million children have fled Ukraine in less than two weeks of war, James Elder, a spokesperson for UNICEF, said on Tuesday, calling it “a dark historical first.”

Since the start of the Russian invasion Feb. 24, 2 million people have fled Ukraine, according to the U.N. — more than those who left Syria in the first three years of the civil war there.

“We have not seen a refugee crisis of this speed and scale since World War II,” said Elder, “And this is a children crisis.”

About 1 in 7 Ukrainian children have already left the country. At least 29 have been killed, by bombings or by mortar shells, in the street as they tried to escape, a figure that humanitarian groups say is almost certainly an underestimate. Since the start of the conflict, UNICEF estimates that 4,000 babies have been born in Ukraine “including dozens in makeshift maternity wards and underground shelters,” the agency said in a statement to The New York Times.

“Despite the best efforts of doctors and midwives, they are setting up ICUs or surgeries in basements and bunkers — no way it can be as safe as where a child should be born,” said Elder.

In some areas, families are living without electricity and water, with children exposed to the cold, disease, hunger and thirst. Families from Mariupol recounted that some children drank water from water heaters, said Elder.

Catherine Russell, the executive director of UNICEF, called the situation of children in Ukraine “a moral outrage,” and said that countless of them were traumatized.

Many of the children fleeing are unaccompanied, the U.N. said. Their parents or family members stayed in Ukraine, or were killed, Elder said, adding that he had met a mother who was traveling with her three children and three children of her sister, with whom she had lost contact.

About 100,000 children, half of them with disabilities, are living in institutions and boarding schools that are at risk, as schools, orphanages, homes and hospitals had all come under attack, the U.N. said.

Most of the Ukrainian refugees in other countries are women and children, because men between the ages of 18 and 60 are not allowed to leave.

This article originally appeared in The New York Times.