According to the United Nations, more than two million Ukrainians have fled since Russia began its invasion on February 24. That is more than those who left Syria in the first three years of its civil war.

The U.N. estimates that the total number of Ukrainian evacuees will grow to more than four million. Many believe this could become Europe’s most serious humanitarian crisis since World War II.

Among the evacuees, one million of them have been children, according to James Elder, a spokesman for UNICEF, who called it “a dark historical first.” That means about one-in-seven Ukrainian children have left the country.

“We have not seen a refugee crisis of this speed and scale since World War II,” Elder told The New York Times, adding, “this is a children crisis.”

According to the U.N., many of the children fleeing are unaccompanied. Elder said that many of their parents or family members are staying in Ukraine or were killed. Elder added that he met a mother traveling with her three children and her sister’s three children, with whom she had lost contact.

Ukraine’s neighbor, Poland, has taken in more than half of the two million Ukrainian refugees. Poland is working to open new shelters as Ukrainians pour into the country.

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Humanitarian corridors have been opened and allowed hundreds of people to flee from cities that have seen heavy fighting, such as the city of Sumy in eastern Ukraine.

The corridor leading out of Sumy was the first major humanitarian corridor to be established, despite shooting in the nearby areas around the route, according to Dmytro Zhyvytsky, the governor of the region. Ukrainian government officials have said that thousands of people will continue to be evacuated from Sumy.

The government of Ukraine has also established humanitarian corridors to move people out of the areas around Kyiv, which has seen heavy fighting. A train has been shuttling women and children out of the Kyiv area.

According to Ukrainian officials, some efforts to establish corridors out of other cities have been abandoned because of Russian attacks on the routes.

The Russian government proposed setting up humanitarian corridors out of several Ukrainian cities that would lead to Russia and Belarus. According to Interfax, Russia wanted civilians fleeing the Ukrainian cities of Kyiv, Chernigov, and Kharkiv to travel to Russia, some via Belarus.

Ukrainian President Volodymyr Zelenskyy rejected the Russian proposal, saying Ukrainian citizens will not be going to “occupied territory” in Russia and Belarus.

Ukraine’s Deputy Prime Minister, Iryna Vereshchuk, echoed the same sentiments in a video statement.

“This is [an] unacceptable option of creating humanitarian corridors [only in Russia and Belarus]. Our people from Ivankiv, Dymer, Vyshhorod, Kyiv won’t go to Belarus to fly later to Russia,” said Vereshchuk.

The Ukrainian ambassador to the United Nations, Sergiy Kyslytsya, also emphasized the country’s opposition to Russia’s proposal at a U.N. Security Council meeting.

He said Russia had “undermined arrangements” for humanitarian corridors by insisting all routes would go through Russia or Belarus.