Over 10,000 new North Korean workers have registered in Russia since September, the paper said, citing records from the Russian Interior Ministry.

Russia’s action potentially violates UN sanctions to reduce cash flows to North Korea and put pressure on Pyongyang to give up its nuclear weapons, the Journal reported, citing US officials.

Labour Ministry records obtained by the Journal showed that a minimum of 700 new work permits have been issued to North Koreans in Russia this year, the paper said.

U.N. officials are probing potential violations of the sanctions, which contain narrow exceptions, WSJ reported citing sources.

Russia’s foreign ministry did not respond to a request for comment outside regular business hours.

“It’s absolutely clear that Russia needs to do more. Russia says it wants better relations with the United States, so Moscow should prove that by cooperating with us, not working against us, on this urgent threat to all nations,” a US State Department spokesperson told Reuters.

“It is estimated that North Korean labourers in Russia send between US$150 million – US$300 million annually to Pyongyang. Now is the time for Russia to take action: Moscow should immediately and fully implement all the UN sanctions that it has signed on to,” the State Department spokesperson said.

The labour prohibition is a part of a broader array of sanctions aimed at eliminating an important revenue stream for Kim Jong Un’s regime. Most of the money North Koreans earn abroad ends up in government coffers as workers toil in gruelling conditions, the Journal reported.

UN Humanitarian Chief Mark Lowcock visited Pyongyang last month and posted a video online outlining his observations. “One of the things we’ve seen is very clear evidence of humanitarian need here,” he said in the video, posted to his official Twitter account and the UN website.