NATO Chief Mark Rutte said on Friday he understands Washington's need to maintain its own weapons stockpiles, but added he hopes for flexibility to ensure Ukraine continues receiving necessary military support.

"I totally understand for the U.S. to make sure that the stockpiles are at a level that we have the collective defense we need," Rutte told reporters after a change-of-command ceremony that saw U.S. Air Force General Alexus Grynkewich take over as NATO's Supreme Allied Commander Europe.

But "we all hope for a level of flexibility to make sure that also Ukraine has what it needs," Rutte added.