WASHINGTON — U.S. authorities accused a Russian-born citizen of South Africa on Monday of taking part in a scheme to help North Korea evade sanctions aimed at curbing its nuclear weapons program.

The Treasury Department said Vladlen Amtchentev worked with Singapore-based front companies to launder millions of dollars through the U.S. financial system on behalf of sanctioned North Korean banks.

Amtchentev was placed on an international financial blacklist by the Treasury Department's Office of Foreign Assets Contol that lists the 49-year old as residing in Singapore. The action freezes any U.S. assets Amtchentev may have and bars any U.S. organizations from conducting financial transactions with him.

The U.S. filed criminal charges against the front companies in August 2017.

That complaint alleges that Singapore-based Velmur Management Pte Ltd. and Transatlantic Partners Pte. Ltd. laundered funds from North Korean banks that were seeking to procure petroleum products from a sanctioned Russian company. The U.S. government is still seeking the forfeiture of $5 million that was allegedly wired to Velmur. It is also asking the court to impose more than $21 million in penalties against Velmur. Amtchentev is not named in that case.

Exports of oil and petroleum products are heavily restricted under U.N. Security Council resolutions intended to deprive North Korea of funds and resources for its nuclear and ballistic missile programs.

"North Korea depends upon the help of criminals and illicit actors to raise and transfer funds," Treasury Secretary Steven Mnuchin said in a statement. "As part of our commitment to the final, fully verified denuclearization of North Korea, Treasury will continue to enforce and implement sanctions against any actor that seeks to aid the regime's deceptive practices."

Washington's intensification of sanctions against North Korea has become a sticking point in negotiations on ending its nuclear program that have appeared to stall in recent months. North Korea called off planned talks with Secretary of State Mike Pompeo in New York this month that were intended to pave the way for a second summit between President Donald Trump and North Korean leader Kim Jong Un.

Meanwhile cooperation between South Korea and North Korea has grown stronger, leaving Washington out of step with a key ally, although Seoul also says it will continue to enforce sanctions until the North denuclearizes.

The risk of disconnect between the U.S. and South Korea is expected to be discussed Tuesday when U.S. Special Representative for North Korea Stephen Biegun meets in Washington with his South Korean counterpart, Lee Do-hoon.

The State Department said Monday the two officials will discuss ongoing diplomatic efforts, sustained implementation of U.N. sanctions, and inter-Korean cooperation.