China mines 95 percent of the world’s rare earth elements, which have broad commercial and military applications, and are vital to the manufacture of products as diverse as cellphones, large wind turbines and guided missiles. Any curtailment of Chinese supplies of rare earths is likely to be greeted with alarm in Western capitals, particularly because Western companies are believed to keep much smaller stockpiles of rare earths than Japanese companies.

China experts said on Tuesday that Beijing’s assertive stance on rare earths might also signal the ascendance of economic nationalists, noting that the Central Committee of the Communist Party convened over the weekend.

A few rare earth shipments to the West have been delayed by customs officials in recent weeks, said industry officials in China, Japan and the United States. But new restrictions on exports appear to have been imposed on Monday morning.

Industry executives said there had been no signal from Beijing of how long rare earth shipments intended for the West would be held by Chinese customs officials. A few shipments are still being allowed out of the country for reasons that remain unclear: a fourth rare earth industry official said on Wednesday that one of the 32 authorized rare earth exporters in China had been allowed to export one container of rare earths to the West on Tuesday and hoped to be allowed to ship another on Thursday.

China’s official stance remained unclear on Wednesday. In an apparent reference to a report on Tuesday in the official China Daily newspaper, the commerce ministry said the report, predicting a decline of up to 30 percent in rare earth export quotas next year, was “totally groundless and purely false,” and added that no decision had been made yet on future quotas.