WASHINGTON (REUTERS) - The United States on Friday accused Russia of violating UN sanctions on North Korea, citing "credible reports" that Moscow was granting new work permits to North Korean labourers despite Russia's denial of any such actions.

The Trump administration also imposed targeted US 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 programme.

This came on the same day that US Secretary of State Mike Pompeo, less than two months after a landmark US-North Korea summit in Singapore, flew back to the city-state and suggested that continued work on weapons programmes by North Korea was inconsistent with its leader's commitment to denuclearisation.

Russia denied a report by the Wall Street Journal that said Moscow was allowing thousands of fresh North Korean labourers into the country and granting them work permits in a potential breach of UN sanctions.

Over 10,000 new North Korean workers have registered in Russia since September, the newspaper said, citing records from Russia's interior ministry.

Russia's action potentially violates UN sanctions aimed at pressuring Pyongyang to give up its nuclear weapons, the Journal reported, citing US officials.

It also raises further questions about the extent to which North Korea's larger neighbors, including its biggest trading partner, China, are willing to keep up enforcement of US-led international sanctions on Pyongyang.

In the summit with US President Donald Trump on June 12, North Korean leader Kim Jong Un, who is seeking relief from tough sanctions, committed in broad terms to work towards denuclearisation of the Korean peninsula. But Pyongyang has offered no details on how it might go about this, and many experts doubt it will ever give up its nuclear arsenal.

"Credible reports of Russia violating UN Security Council resolutions on North Korean labourers working abroad are deeply troubling," US Ambassador to the United Nations Nikki Haley said later on Friday.

"Talk is cheap - Russia cannot support sanctions with their words in the Security Council only to violate them with their actions," Haley said in a statement.

RUSSIAN DENIAL

Alexander Matsegora, Russia's ambassador to North Korea, denied Moscow had allowed any new workers to enter Russia, saying fresh documents had been issued to laborers already based in Russia who were working on old contracts, Interfax news agency reported.

He said the laborers were allowed to work in Russia until Nov 29, 2019 as their work contracts had been signed before sanctions came into force, Interfax reported. He said 3,500 new work permits had been issued to workers who had signed contracts in Russia before Nov 29, 2017, Interfax reported.

Labour Ministry records obtained by the Journal showed that a minimum of 700 new work permits had been issued to North Koreans in Russia this year, the paper said.

"It is estimated that North Korean laborers in Russia send between US$150 million and US$300 million annually to Pyongyang," a US State Department spokesperson said.

"Moscow should immediately and fully implement all the UN sanctions."

The labour prohibition is a part of a broader array of sanctions aimed at eliminating an important revenue stream for North Korea. Most of the money North Koreans earn abroad ends up in government coffers as workers toil in gruelling conditions, the Journal reported.

A separate report released on Thursday by the non-profit research organization C4ADS said initial restrictions in China and Russia - where around 80 per cent of North Korean laborers are believed to work - appear to have loosened.

In Friday's sanctions announcement, the US 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 Treasury also added Ri Jong Won, the Moscow-based deputy representative of FTB, to its sanctions list and said both Ri and Han should be expelled from Russia.

A separate report released this week by the Seoul-based Asan Institute for Policy Studies said between 2015 and 2017 the Moscow-based Independent Petroleum Company (IPC) sold far more oil to North Korea than what was officially reported.

Russia's ambassador to North Korea also denied Moscow was flouting UN restrictions on oil supplies to North Korea.

In September last year, Reuters found that at least eight North Korean ships that left Russia with a cargo of fuel had headed for their homeland despite declaring other destinations, a ploy that US officials say is often used to undermine sanctions.