The United States on Tuesday imposed sanctions on six entities and four vessels for their alleged involvement in the transport of North Korean coal.

The U.S. Department of the Treasury's Office of Foreign Assets Control announced that it has designated six entities and identified four vessels related to the transport of North Korean coal.

The six new entities include trading or shipping companies based in the North's capital city Pyongyang, China, Hong Kong and Vietnam. 

Treasury Secretary Steven Mnuchin said 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. 

He added that 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 Trump administration, which has about 40 days before the expiration of Trump's term, appears to be stepping up its pressure on North Korea as well as China through sanctions against the North.