The U.S. announced new sanctions on entities and ships it says are helping North Korea sell coal in violation of international restrictions meant to reign in Kim Jong Un’s nuclear weapons program.

The Treasury Department said Tuesday it was sanctioning six entities, including China-based Weihai Huijiang Trade Ltd. as Always Smooth Ltd and Good Siblings Ltd., which are registered in the U.K. Hong Kong-based Silver Bridge Shipping Co-HKG, Korea Daizin Trading Corporation and Vietnam-based Thinh Cuong Co. were also targeted.

The U.S. also identified four vessels related to the transport of North Korean coal, and called on Chinese authorities to “implement and enforce” United Nations resolutions by taking action against “companies, individuals and vessels that engage in” sanctions busting.

North Korea “continues to circumvent the UN prohibition on the exportation of coal, a key revenue generator that helps fund its weapons of mass destruction programs,” said Treasury Secretary Steven Mnuchin. “The North Korean regime often uses forced labor from prison camps in its mining industries, including coal, exploiting its own people to advance its illicit weapons programs.”

The actions come after a senior U.S. official earlier this month stepped up criticism of China for violating UN sanctions, an acknowledgment that a central tenet of U.S. policy toward Pyongyang—the strict enforcement of sanctions to pressure Kim to join denuclearization talks—has largely collapsed.

North Korea jump-started coal exports after shutting down almost all shipments during the first few months of the year while it sought to block the spread of the coronavirus, a UN report alleged over the summer. Imports of goods and commodities by road and freight have also restarted, though not at pre-pandemic levels, a panel monitoring the enforcement of sanctions on North Korea said in a report.