The Treasury Department said it was taking action against three companies based in mainland China, two operating in Vietnam and one based in Hong Kong along with four vessels.

Treasury Secretary Steven Mnuchin said that the firms and ships transported coal out of North Korea in defiance of UN sanctions and despite concerns that the production relies on forced labor in prison camps.

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," Mnuchin said in a statement.

The sanctions block any assets of the companies in the United States and make transactions with the firms or ships subject to prosecution.

The move comes a week after a US official dealing with North Korea sharply criticized China for not doing more to enforce the sanctions.

The official, Alex Wong, acknowledged that North Korea had not taken concrete measures to end its nuclear program despite three meetings between outgoing President Donald Trump and leader Kim Jong Un.

Deputy Secretary of State Stephen Biegun, who is in charge of diplomacy with North Korea, is visiting South Korea this week on what could be his last in-person consultations before Trump leaves office.