China failed to send home North Korean workers by a December deadline in violation of United Nations sanctions, a senior U.S. official said Wednesday, adding that this was why Washington had blacklisted two entities involved in Pyongyang's labor exports.

A 2017 U.N. Security Council resolution, which China backed, demanded that all countries repatriate all North Korean workers by Dec. 22 to stop them from 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 whom 50,000 were in China and 30,000 in Russia.