UPDATE 2-U.S. hits Russian bank with sanctions for N.Korea-related activity

(Adds details on sanctions in paragraph 3, letter by Republicans, Russia comment in paragraphs 9 to 12)

WASHINGTON, Aug 3 (Reuters) - The United States on Friday imposed sanctions on a Russian bank it said had facilitated a transaction with a person blacklisted by Washington for involvement with North Korea's nuclear weapons program.

The move comes as U.S. Secretary of State Mike Pompeo presses Southeast Asian countries during meetings in Singapore to maintain sanctions to pressure Pyongyang, which is in talks with the United States about dismantling its nuclear program.

Sanctions by the United States and the United Nations Security Council, which include a ban on exports of coal, iron, lead, textiles and seafood from North Korea, and caps on imports of oil and refined petroleum products, are aimed at choking off funding for Pyongyang's nuclear and ballistic missile programs.

The U.S. Treasury Department said Moscow-based Agrosoyuz Commercial Bank had conducted "a significant transaction" for Han Jang Su, the Moscow-based chief representative of Foreign Trade Bank (FTB), North Korea's primary foreign exchange bank.

"The United States will continue to enforce UN and U.S. sanctions and shut down illicit revenue streams to North Korea," U.S. Treasury Secretary Steven Mnuchin said in a statement.

The Treasury also added Ri Jong Won, the Moscow-based deputy representative of FTB, and said both Ri and Han should be expelled from Russia under U.N. resolutions meant to pressure Pyongyang over its nuclear program.

It also designated what it said were two FTB front companies, China-based Dandong Zhongsheng Industry & Trade Co Ltd and Korea Ungum Corporation, which is based in North Korea.

The United States has kept up its pressure campaign on North Korea even as U.S. President Donald Trump has pursued talks with the country's leader, Kim Jong Un, to denuclearize.

Questions have arisen over Pyongyang's commitment to give up its nuclear program after U.S. spy satellite material detected renewed activity at the factory that produced North Korea's first intercontinental ballistic missiles capable of reaching the United States.

In a letter to Trump on Friday, Republican senators Cory Gardner, Lindsey Graham, Marco Rubio and Dan Sullivan urged the administration to tighten sanctions.

"Mr President, we believe that the time to ramp up maximum pressure against North Korea is now, if we are to peacefully achieve the goal of the complete, verifiable, and irreversible denuclearization of the Korean Peninsula in the near future," the senators wrote.

Russia on Friday denied a Wall Street Journal report that it was allowing thousands of new North Korean laborers into the country and granting them work permits in a potential breach of U.N. sanctions. (Reporting by Susan Heavey, Tim Ahmann and Lesley Wroughton in Washington; Editing by Jonathan Oatis and James Dalgleish)