GANZHOU, China — 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 Donald Trump’s sharp increase in tariffs that started 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% 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.

The so-called heavy rare-earth metals covered by the export suspension are used in magnets essential for many kinds of electric motors. These motors are crucial components of electric cars, drones, robots, missiles and spacecraft. Gasoline-powered cars also use electric motors with rare-earth magnets for critical tasks such as steering.

The metals also go into the chemicals for manufacturing jet engines, lasers, car headlights and certain spark plugs. And these rare metals are vital ingredients in capacitors, which are electrical components of the computer chips that power artificial intelligence servers and smartphones.

In a potential complication, China’s Ministry of Commerce has barred Chinese companies from having any dealings with an ever-lengthening list of American companies, particularly military contractors.

MP Materials owns the sole rare-earths mine in the United States — the Mountain Pass mine in the California desert near the Nevada border — and hopes to start commercial production of magnets in Texas at the end of the year.

A few Japanese firms keep rare-earth inventories of more than a year’s supply, having been hurt in 2010, when China imposed a seven-week embargo on such exports to Japan during a territorial dispute. But many U.S. firms keep little or no inventory because they do not want to tie up cash in stockpiles of costly materials.