The US and the EU have warned that a shortage of rare earth metals from China could harm their economies.
Officials and industrialists in Washington and Berlin said a lack of the minerals, 90% of which come from China, would have severe repercussions.
The metals are essential for making many hi-tech products and some are used by the US weapons industry.
China says it is cutting production to protect the environment and preserve its own supply.
It has denied it is using its near-monopoly improperly and says there will not be drastic cuts in exports.
Rare earth metals are scarce minerals that have particular properties, such as being magnetic or shining in low light.
This makes them particularly useful in some new technologies, such as solar panels or electric cars or light-weight batteries.
German companies have lobbied the government to say that they are now finding it hard to get hold of these essential minerals, because China produces virtually all of them.
German economy minister Rainer Bruederle said his country was being severely affected.
The concerns about a monopoly are now being echoed in Washington, where the Department of Energy has said firms are worried about shortages. One talked of a "supply crunch".
In the US, efforts are being made to increase production, but this cannot be done in the near future.