The decision left officials on both sides of the Atlantic scrambling to find out how long the pause might last.

Lawmakers, former diplomats and experts questioned the Trump administration’s decision to halt the supply of some air-defense weapons to Ukraine, saying the move risks prolonging the war with Russia and leaving the country more vulnerable to stepped-up missile and drone barrages.

The move "was made to put America’s interests first," a White House spokeswoman said, as U.S. munitions have been depleted in recent years.