China has suspended exports of a wide range of critical minerals and magnets, threatening to choke off supplies of components central to automakers, aerospace manufacturers, semiconductor companies and military contractors around the world.

Shipments of the magnets, essential for assembling everything from cars and drones to robots and missiles, have been halted at many Chinese ports while the Chinese government drafts a new regulatory system. Once in place, the new system could permanently prevent supplies from reaching certain companies, including American military contractors.

The official crackdown is part of China’s retaliation for President Trump’s sharp increase in tariffs that started on April 2.

On April 4, the Chinese government ordered restrictions on the export of six heavy rare earth metals, which are refined entirely in China, as well as rare earth magnets, 90 percent of which are produced in China. The metals, and special magnets made with them, can now be shipped out of China only with special export licenses.

But China has barely started setting up a system for issuing the licenses. That has caused consternation among industry executives that the process could drag on and that current supplies of minerals and products outside of China could run low.

If factories in Detroit and elsewhere run out of powerful rare earth magnets, that could prevent them from assembling cars and other products with electric motors that require these magnets. Companies vary widely in the size of their emergency stockpiles for such contingencies, so the timing of production disruptions is hard to predict.