China first responded on April 3 to Trump's initial salvo of 54 percent levies on its exports by placing export restrictions on rare earth elements, which are key for the fighter aircraft that will form the backbone of the U.S. Air Force's next-generation fleet.

Shipments of seven rare earths placed on an export control list last week by Beijing have ground to halt, three sources said, raising the risk of shortages overseas as Chinese exporters begin the long,