US Secretary of State Mike Pompeo called Saturday for pressure to be maintained on North Korea as a UN report warned Pyongyang is circumventing tough sanctions imposed over its nuclear weapons programme.

In Singapore ahead of a major security forum, Pompeo said he had urged other countries to strictly enforce United Nations sanctions against North Korea , which appears to have made slow progress towards denuclearization following a pledge at a landmark June summit.

He singled out Russia after reports suggested they were breaching sanctions and said China -- one of Pyongyang's few allies -- had vowed to enforce the measures.

The US top diplomat said he had "emphasized the importance of maintaining diplomatic and economic pressure on North Korea to achieve the final, fully verified denuclearization that DPRK has agreed to", using the initials of the North's official name.

"I must say from my meetings here, the world is united in seeing this achieved," Pompeo told a press conference.

"We're determined to do it, Chairman Kim is committed to doing it. I'm optimistic that we will get this done."

As ministers gathered for a photo at the meeting in the city-state, Pompeo went over to greet his North Korean counterpart Ri Yong Ho, with the pair shaking hands, exchanging smiles and a few words.

Pompeo has travelled to the North several times in recent months, including in July as he sought to flesh out Pyongyang's denuclearization commitment.

Evading sanctions 

His comments came as a UN report said Friday that North Korea has resorted to a "massive increase" of illegal ship-to-ship transfers of oil products at sea to evade sanctions.

The 62-page report sent to the Security Council also listed violations of a ban on North Korean exports including coal, iron and seafood that generate millions of dollars in revenue for the reclusive regime.

At the summit with President Donald Trump in June, the North's leader Kim Jong Un signed up to a vague commitment to "denuclearisation of the Korean Peninsula" -- a far cry from long-standing US demands for complete, verifiable and irreversible disarmament.

While there have been small signs of progress, news reports indicate Pyongyang is continuing to build rockets and there have been concerns that some member states are relaxing the enforcement of sanctions.

There has been no clear indication how long it will take the North to disarm or how exactly it will do so, and Pompeo said in Singapore the timing would be up to Kim.

"The ultimate timeline for denuclearisation will be set by Chairman Kim, at least in part," he told local broadcaster Channel NewsAsia in an interview. "The decision is his."

At Saturday's press conference, Pompeo said the US was "heartened" by Pyongyang's recent return of the remains of dozens of American soldiers killed during the Korean War.

But he also warned that Washington would take the infringement of sanctions by other countries "very seriously".

"We have seen reports that Russia is allowing for joint ventures with North Korean firms and granting new work permits to North Korean guest workers," Pompeo said.

"If these reports are proven accurate, and we have every reason to believe that they are, that would be in violation" of UN sanctions, he added.

Oil transfers

Pompeo said that during his meetings in Singapore, he called specifically for countries to enforce sanctions by stopping ship-to-ship transfers of petrol.

Cutting off oil and fuel to the North would require enforcement primarily by China, which supplies most of North Korea 's energy needs, but also by Russia, which delivers some oil to Pyongyang.

The US top diplomat said that in talks Friday, Foreign Minister Wang Yi was clear on China's "continued commitment" to enforce sanctions.

Pompeo also held talks with his counterparts from South Korea and Japan.

Saturday's forum, hosted by the Association of Southeast Asian Nations (ASEAN), brings together top diplomats from 26 countries and the European Union for talks on political and security issues in the Asia-Pacific.

Wang also met with the North's Ri in Singapore and praised Pyongyang's "efforts in promoting the denuclearisation process", China's official Xinhua news agency said.

After attending the Singapore meeting, Ri will head to Iran for an official visit, according to North Korea 's official news agency KCNA.

Iran, whose nuclear drive has also sparked international alarm, and North Korea are reported to have a history of cooperation in military and missile technology.