China has managed to hit the U.S. where it hurts the most in response to the Trump administration’s tariffs, with restrictions on global exports of certain rare-earth elements that the U.S. has little ability to produce or refine.

Beijing has a virtual monopoly on rare earth minerals—the materials that power everything from military planes to your electric toothbrush.

Shares of Boeing fell in early trading Tuesday following a report that China has halted delivery of its jets to airlines in that country as part of the escalating trade war between the country and the United States.

Boeing slumped as much as 3% on Tuesday. The aerospace stock is now down 12% for the year. Why: Shares are slipping after China reportedly ordered domestic airlines to stop purchasing aircraft products from the US.