Industry officials said that mainland China’s customs agency had notified companies that they were not allowed to ship to Japan any rare earth oxides, rare earth salts or pure rare earth metals, although the shipments are still allowed to go to Hong Kong, Singapore and other destinations. But no ban has been imposed on the export to Japan of semi-processed alloys that combine rare earths with other materials, the officials said. China has been trying to expand its alloy industry to create higher-paying jobs in mining areas, instead of exporting raw materials for initial processing.

A senior Japanese foreign ministry official, who declined to be named, said that the Japanese government had not yet received any notice from China regarding the suspension. The official said, however, that the government had repeatedly asked China to not restrict its exports of rare earth elements, citing the severe consequences such a move would have on global production and trade.

Toyota, which makes the Prius hybrid car, had not yet received any information on an embargo and was unable to comment, a spokesman for Toyota in Tokyo, Masami Doi, said.

Japan has been the main buyer of Chinese rare earths for many years, using them for a wide range of industrial purposes, like making glass for solar panels. They are also used in small steering-control motors in conventional gasoline-powered cars as well as in motors that help propel hybrid cars like the Prius.

American companies rely mostly on Japan for magnets and other components using rare earth elements, as the United States’ manufacturing capacity in the industry became uncompetitive and mostly closed over the last two decades.

The Chinese halt to exports is likely to have immediate repercussions in Washington. The Committee on Science and Technology of the House of Representatives was scheduled Thursday to review a detailed bill to subsidize the revival of the American rare earths industry. The main American rare earth mine, in Mountain Pass, Calif., closed in 2002, but efforts are under way to reopen it.