The United States has imposed restrictions on exports to China's biggest chipmaker SMIC after concluding there is an "unacceptable risk" equipment supplied to it could be used for military purposes.

Suppliers of certain equipment to Semiconductor Manufacturing International Corporation will now have to apply for individual export licenses, according to a letter from the Commerce Department dated Friday and seen by Reuters.

The latest move marks a shift in U.S. policy from earlier this year, when applicants seeking "military end user” licenses to sell to SMIC were told by the Commerce Department that the licenses weren't necessary, according to three people familiar with the matter.