KYIV (Reuters) - Fatigue among donors could compound the issues created by the mass displacement of Ukrainians after Russia's invasion, making the problem more expensive to deal with in the long run, the U.N. migration agency chief said in Kyiv on Thursday.

       Russia's full-scale invasion of Ukraine in February 2022 triggered the biggest refugee crisis in Europe this century, with 6.5 million Ukrainian refugees having been recorded globally and 3.7 million displaced within Ukraine, according to U.N. data.

       The U.N. also says that over 14 million Ukrainians are in need of some form of humanitarian assistance.

       "If the donor community walks away right now, it's not going to fix the problem - actually the problem could become compounded, and become more expensive," International Organization for Migration Director General Amy Pope told Reuters at a joint interview with local media in Kyiv.

       "Ultimately, we are not only talking about a much more significant response, but (also) about greater loss of life, greater instability, greater regional instability."

       Pope cited as an example the current delays over provision of aid to Ukraine by the United States, the biggest single state donor to Kyiv since Russia's invasion.

       White House national security adviser Jake Sullivan said during a trip to Kyiv last month that the U.S. aid package that has been blocked by Republicans for months would "get to Ukraine" and vowed that Washington's support would continue.

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       "It's absolutely wrong to think the situation has stabilised by any stretch," Pope said at the end of a five-day trip across Ukraine, during which she met officials in regions affected by Russian airstrikes, as well as meeting President Volodymyr Zelenskiy in Kyiv.

       The IOM chief said that during their conversation Zelenskiy had been particularly interested in guaranteeing the provision of education, and particularly in ensuring that schools had enough bomb shelters for their children.

       She said that people were still leaving areas which had come under more intense bombardment recently, citing the southern port city of Odesa as an example.

       Pope encouraged the creation of favourable conditions to attract refugees back to Ukraine. She said ensuring safety was the primary criterion for this, but that solutions such as identifying business opportunities for returnees and encouraging firms to employ them could also help.

       (Reporting by Max Hunder; Editing by Alison Williams)