The Treasury said Primorye Maritime Logistics Co. and Gudzon Shipping Co. own a tanker, the M/V Patriot, which conducted ship-to-ship transfers of oil to North Korea tankers twice earlier this year.

That violated a UN-backed embargo on doing business with North Korea (DPRK), part of an effort to pressure Pyongyang to give up its nuclear weapons and ballistic missiles programs.

"Ship-to-ship transfers with North Korea-flagged vessels from Russia or elsewhere of any goods being supplied, sold, or transferred to or from the DPRK are prohibited under the UN Security Council resolutions," Treasury Secretary Steven Mnuchin said in a statement.

"Consequences for violating these sanctions will remain in place until we have achieved the final, fully verified denuclearization of North Korea."

The Treasury listed five other vessels owned by Gudzon under the sanctions announcement, which freezes any of the companies' assets in US jurisdiction and severely restricts their access to the global financial system.

In a separate announcement, the Treasury blacklisted two companies and two individuals whom it said were involved in helping another firm, Divetechnoservices, get around sanctions it was hit with in June.

Divetechnoservices and three officials of the firm were originally sanctioned for supplying and supporting the government's underwater capabilities in monitoring and hacking subsea communications cables around the world.