The United States has blacklisted six companies, including several based in China, and four ships accused of illicit exports of North Korean coal, the Treasury Department said on Tuesday.
The United Nations Security Council banned North Korean coal exports in 2017. The 15-member body has unanimously boosted sanctions on North Korea since 2006 in a bid to choke off funding for Pyongyang's nuclear and ballistic missile programs.
"The DPRK (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," US Secretary Steven Mnuchin said in a statement.
"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," he said.
An annual report to the UN Security Council by independent sanctions monitors earlier this year said North Korea continued to flout council resolutions "through illicit maritime exports of commodities, notably coal and sand" in 2019, earning Pyongyang hundreds of millions of dollars.
The US action freezes any US assets of those sanctioned and generally bars Americans from dealing with them.
When asked about the action, Chinese Foreign Ministry spokesman Zhao Lijian said China fulfilled its international obligations and opposed countries implementing unilateral sanctions against Chinese companies.
"China will continue to firmly protect Chinese companies' and individuals' legitimate and legal rights and interests," Zhao told a daily news briefing in Beijing on Wednesday.
The blacklisted ships - Calm Bridge, Asia Bridge, Lucky Star and Star 18 - "loaded coal directly from North Korean ports and transported their cargo throughout the region," the US Treasury Department said.
The shipping companies sanctioned are China-based Weihai Huijiang Trade Ltd, Always Smooth Ltd, and Good Siblings Ltd. Always Smooth and Good Siblings are also registered in Britain, the Treasury Department said.
It also designated Hong Kong-based Silver Bridge Shipping Co-HKG, Vietnam-based Thinh Cuong Co Ltd and Korea Daizin Trading Corporation, which operates in North Korea and Vietnam.
North Korean leader Kim Jong Un and US President Donald Trump have met three times since 2018, but failed to make progress on US calls for Pyongyang to give up its nuclear weapons and North Korea's demands for an end to sanctions.