Russia says it is working on retaliatory measures against the United States after Washington imposed sanctions on a Russian company and its head for helping North Korea evade United Nations sanctions.

"These restrictions have been introduced without providing any evidence and on the basis of unfounded accusations," Russian Deputy Foreign Minister Sergei Ryabkov said on August 16.

The U.S. Treasury Department said on August 15 that it was taking actions against three shipping companies based in China, Singapore, and Russia, as well as the head of the Russian shipping firm, Profinet Pte.

The sanctions block any assets the three companies may have in U.S. jurisdictions and bar U.S. citizens from doing business with them.

Despite having opened direct talks with Pyongyang, Washington continues to press for full compliance with existing international sanctions against North Korea to keep pressure on Pyongyang to end its programs for nuclear weapons and ballistic missiles.

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

The Treasury blacklisted Profinet Pte. for providing port services like loading and refueling to sanctioned North Korean-flagged ships at three eastern Russian ports.

It also said the company's director-general, Vasily Kolchanov, "was personally involved" in deals with the North Koreans.

Kolchanov told Reuters the sanctions against him and his firm were illegal, and said he only helped prepare documents for the North Korean vessels.

Kolchanov insisted he had worked with the ships as a transport agent, but was not involved in trading any goods.

The Treasury also accused China-based Dalian Sun Moon Star International Logistics Trading Co and its Singapore-based affiliate, SINSMS Pte., of falsifying documents to facilitate shipments of alcohol and cigarettes to North Korea.

China's Foreign Ministry said Beijing had always strictly enforced UN Security Council resolutions on North Korea and did not allow Chinese firms or individuals to engage in activities that violated the UN sanctions.

With reporting by Reuters and Interfax