The news comes as the UN Security Council ratified new sanctions against North Korea after its sixth nuclear test since 2006. The 15-member Council, including permanent members Russia and the U.S., unanimously passed the sanctions resolution on Monday banning textile exports and capping fuel imports.

The Justice Department and Treasury Department were reportedly motivated by Pyongyang’s increased trade with Moscow when it blacklisted four Russian citizens and one Russian company in August.

The Post cites U.S. court documents that describe a web of “front companies” formed by Russian citizens to specifically hide deals with North Korea.

A Treasury Department official points to China’s willingness to “increase the pressure” on North Korea and a lack of clarity on whether “the Russians are willing to go along with that,” the Post reported.

“Russia is now a player in this realm,” said Anthony Ruggiero, now a fellow with the Foundation for Defense of Democracies Washington think tank.