Exchanges have taken place before, but this deal, involving big numbers on both sides, comes as Trump prepares to take over in Washington, potentially endangering American aid to Kyiv.

Russia’s once-dominant gas supply to Europe via Ukraine, which flowed for decades, is set to end on New Year’s Day with the collapse of a contract between the two warring countries that paid out billions to Moscow in gas revenue and to Kyiv in transit fees.

Biden's administration is working quickly to spend all the money it has available to help Kyiv fight off Russia before President-elect Donald Trump takes office in three weeks.