WARSAW, Poland (AP) — Warsaw’s mayor is appealing for international help as the city becomes overwhelmed by refugees, with more than a tenth of all those fleeing the war in Ukraine arriving in the Polish capital.

Some seek to wait out the war or settle in the city, while others merely use Warsaw as a transit point to head further west, turning its train stations into crowded hubs where people are camping out on floors.

“We are dealing with the greatest migration crisis in the history of Europe since World War II. … The situation is getting more and more difficult every day,” Mayor Rafal Trzaskowski said, adding that “the greatest challenge is still ahead of us.”

The welcome Warsaw has given Ukrainians as the neighboring nation struggles to resist Russia’s invasion is wholehearted. Across the city, people have mobilized to help. They are taking Ukrainians into their homes, gathering donations and volunteering at reception centers. City monuments and buses fly Ukraine’s blue-and-yellow flag in solidarity.

But the challenge is enormous. Much of the burden so far is being carried by volunteers taking time off work, a situation not sustainable in the long run.

Mr. Trzaskowski noted on Friday that child psychologists, in one example, had been volunteering to help refugees but soon will need to return to their jobs.

Housing is also a growing problem. When the war began, 95 percent of Ukrainians arriving in Warsaw were people who already had friends or family here and were taken in by them. Today that group is 70 percent of the new arrivals meaning that 30 percent of them “need a roof over their heads” and other support, the mayor said Friday.

The decline in the city’s ability to absorb a massive number of new arrivals comes as the people fleeing war are those who have witnessed greater trauma than those who arrived earlier, or who are more vulnerable.