WASHINGTON (Reuters) - The United States on Thursday slapped sanctions on two new companies it accused of being involved in exporting forced labor from North Korea and warned countries to send home any remaining North Korean workers. The U.S. Treasury Department in a statement said it blacklisted Russian construction company Mokran LLC and Korea Cholsan General Trading Corp, 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 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. Russian Foreign Ministry spokeswoman Maria Zakharova told reporters in January that around 1,000 North Korean workers were still in Russia. She said it had proved impossible for all of the workers to leave by the deadline because of limited transport options. Russian Interior Ministry statistics released in January showed that the number of tourist and student visas issued by Russia to North Koreans surged last year. Experts suspect that many of those issued tourist visas are working illegally in Russia. The Treasury on Thursday said that under Korea Cholsan General Trading Corp’s name, a network of Russian companies sought and received work permits for North Korean workers to enter and work in Russia. Mokran LLC also sought and received work permits for North Korean workers to enter and work in Russia, the Treasury said. Moscow and Beijing pushed unsuccessfully in the United Nations in December for a raft of sanctions on North Korea to be lifted.