Malcolm Chalmers, deputy director-general of the Royal United Services Institute military think tank, told CBS News in March that about 20% of Ukraine's military hardware is supplied by the U.S., with 25% provided by Europe and the rest of the world, and 55% produced in Ukraine.

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.

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