China’s Ministry of State Security says it is cracking down on alleged smuggling of rare earths materials that it says threaten national security.

China’s exports of rare-earth products jumped in June, pointing to a potential pickup in magnet supplies after government-imposed curbs that proved to be Beijing’s most powerful weapon in its trade war with the Trump administration.

Beijing uses its near-monopoly on critical minerals to win trade concessions. Can the U.S. find alternate supplies?