The U.S. indicted and sanctioned two Chinese nationals accused of helping North Korean hackers launder around $100 million in stolen virtual currency, alleging Pyongyang’s hackers are receiving outside help and providing a window into what officials described as an increasingly important revenue stream for the regime’s nuclear weapons program.

North Korean hackers were responsible for the theft of around $300 million from four virtual currency exchanges, including a previously unknown 2018 breach that resulted in $235 million...