As the war in Ukraine grinds on, the tide of people seeking refuge from Russia’s assault is rapidly developing into Europe’s biggest wave of migration since World War II, and cracks in the region’s solidarity are beginning to show.

The number of refugees escaping Ukraine — the vast majority arriving in Eastern Europe — could climb to 4 million in a matter of days, according to officials close to the situation, who requested to not be identified. That would match the top overall projection of migrants by United Nations agencies before President Vladimir Putin started the invasion on Feb. 24.

"We have never been in such a situation,” Polish President Andrzej Duda said Thursday during a briefing with U.S. Vice President Kamala Harris. "We are trying to cope with it, but if we don’t get international assistance, it may end up being a humanitarian catastrophe.”