The United States on Thursday slapped sanctions on two companies it accused of being involved in exporting forced labor from North Korea and warned countries still using workers from the country to send them home.

The U.S. Treasury Department in a statement said that it blacklisted Russian construction company Mokran LLC and Korea Cholsan General Trading Corporation, a North Korean company operating in Russia

“North Korea has a long history of exploiting its citizens by sending them to distant countries to work in grueling conditions in order to financially support Pyongyang and its weapons programs,” Treasury Secretary Steven Mnuchin said in the statement.

“Those countries still hosting North Korean workers must send these workers home,” he added.

A 2017 U.N. Security Council resolution demanded that all countries send home all North Korean workers by Dec. 22 last year to stop them earning foreign currency for North Korea’s nuclear and ballistic missile programs.

The United States has estimated Pyongyang was earning more than $500 million a year from nearly 100,000 workers abroad, of which some 50,000 were in China and 30,000 in Russia.

Russia’s ambassador to Pyongyang was quoted by Russian media in May as saying that Moscow’s efforts to repatriate all North Korean workers, as required by a U.N. deadline, had been hampered by the coronavirus outbreak and that the closure of North Korea’s borders meant workers had been sent home.

In January, Russia said it missed a United Nations deadline to repatriate the workers due to what it called objective difficulties, but said it was complying with U.N. sanctions on Pyongyang.