United States Secretary of State Mike Pompeo yesterday warned Russia and the international community not to violate global sanctions on North Korea, as he stressed the importance of maintaining pressure on Pyongyang to denuclearise.

The warning, made at his press conference on the sidelines of Asean-related meetings, came a day after his call on Asean to maintain diplomatic and economic pressure on North Korea amid concerns that Pyongyang is making slow progress towards denuclearisation.

Mr Pompeo addressed Wall Street Journal reports that Russia was entering into joint ventures with North Korean firms and granting new work permits to North Korean guest workers, saying that the US had every reason to believe these reports were accurate.

If true, these actions would be in violation of United Nations Security Council Resolution 2375 that imposed sanctions on North Korea.

He said: "We want to remind every nation that has supported these resolutions and that this is a serious issue and something that we will discuss with Moscow.

"We expect the Russians and all countries to abide by the UN Security Council resolutions and enforce sanctions on North Korea. Any violation that detracts from the world's goal of finally fully denuclearising North Korea is something that America will take very seriously."

Mr Pompeo also called on Asean to strictly enforce all sanctions, including the complete shutdown of illegal ship-to-ship transfers of petroleum destined for North Korea.

"It's worth remembering this isn't just an American security goal... Our partners and allies in Asean know how important the denuclearisation of North Korea is for their own security," Mr Pompeo said.

He added that it was important to maintain the pressure to "achieve the final, fully verified denuclearisation of North Korea, as agreed to by Chairman Kim (Jong Un)".

On denuclearisation, he said: "We're determined to do it. Chairman Kim is committed to doing it. I am optimistic that we will get this done in the timeline."

The US' top diplomat said that he did not have any formal meetings with his North Korean counterpart here during Asean-related meetings.

North Korean Foreign Minister Ri Yong Ho, who is also in Singapore for the Asean Regional Forum, met Chinese Foreign Minister Wang Yi on Friday.

A European Union official confirmed that the EU high representative for foreign affairs Federica Mogherini also had an exchange with Mr Ri in Singapore in which they discussed denuclearisation.