HONG KONG — ZTE is one of China’s few truly international electronics firms. Yet American companies will now need special permission to sell to it.

The company, which makes smartphones, was found to have violated American sanctions against Iran by selling United States-made goods to the country, according to a Commerce Department statement on Monday. As a result, ZTE will be blocked from buying any technology from American companies without a special license.

ZTE planned to “illicitly re-export controlled items to Iran in violation of U.S export laws,” the Commerce Department said. The sanctions against Iran, many of which were recently lifted, were intended to restrict Iran’s nuclear work.

The export controls against ZTE are unusual because such actions are rarely taken against such large companies. The action underscores how important the push is by the United States to gain China’s cooperation in embargoes intended to combat nuclear proliferation.