Ukrainian President Volodymyr Zelenskyy paid tribute on Monday to U.S. nationals who have fought in the 15-month-old war against Russian invaders, saying they were devoted to upholding the values of freedom.

"We Ukrainians will always be grateful to the U.S. and every American for extraordinary support," Zelenskyy, standing outside his office in Kyiv, said in English in a video message marking the U.S. Memorial Day holiday.

"When we remember the warriors who sacrificed their lives in the name of freedom, we recall the reason we are alive, why we live the way we like and why our free nations stand."

Americans have been among thousands of foreign nationals signing up for tours of duty in Ukraine in various units.

There are no official figures for the numbers who have died in the conflict, but media and other reports put the number of U.S. casualties at nearly 20.