Secretary of State Mike Pompeo warned Russia and other countries to respect international sanctions on North Korea on Saturday or risk undermining the pressure on Pyongyang to denuclearize.

His remarks followed a new United Nations report that found North Korea has not stopped its nuclear and missile programs and is violating U.N. sanctions, including through illicit ship-to-ship transfers of oil.

NBC News reported in June that U.S. intelligence agencies believe Pyongyang has increased its production of fuel for nuclear weapons in recent months. In separate reporting experts told NBC that new satellite images revealed that North Korea had continued to expand a nuclear research center.

At a security conference in Singapore, Pompeo told reporters that the U.S. has new, credible reports that Russia is violating U.N. sanctions by allowing joint ventures with North Korean companies and issuing new permits for North Korean guest workers.

“Any violation that detracts from the world’s goal of finally, fully denuclearizing North Korea would be something that America would take very seriously,” Pompeo said.

But at the same time, he oversaw the handover of a letter to North Korean leader Kim Jong Un from President Donald Trump and exchanged pleasantries with the North's top diplomat.

Despite his warnings, Pompeo also stressed that he was optimistic North Korea would denuclearize.

"Chairman Kim is committed to doing it,” he told reporters. “The process of achieving denuclearization of...the North Korean peninsula is one that I think we have all known would take some time, but I must say from my meetings here the world is united in seeing this achieved.”

North Korean Foreign Minister Ri Yong Ho, meanwhile, greeted Pompeo with a smile, but then delivered a scathing attack on the Trump administration for approaching the negotiation poorly by insisting on sanctions enforcement.

Ri said North Korea would not be forced into acting unilaterally and demanded that the U.S. undertake "confidence building" measures if the negotiation was to be successful.

After Pompeo warned anew that no sanctions would be lifted until North Korea fully and finally denuclearizes, Ri told the annual ASEAN Regional Forum that the North would not be bullied into concessions.

"Confidence is not a sentiment to be cultivated overnight," he said.

Ri accused elements of the U.S. government of going against Trump's wishes by taking a hard line on sanctions.

"What is alarming, however, is the insistent moves manifested within the U.S. to go back to the old, far from its leader's intention," he said.

While Pompeo said he did not meet with the North Koreans at the summit, he later tweeted that he had a chance to speak with Ri and posted a picture of the two smiling and shaking hands. “We had a quick, polite exchange,” he wrote.

I had the chance to speak with my #DPRK counterpart FM Ri Yong Ho @asean today. We had a quick, polite exchange. Our US delegation also had the opportunity to deliver @Potus reply to Chairman Kim’s letter. pic.twitter.com/1DiR6UDj4Q — Secretary Pompeo (@SecPompeo) August 4, 2018

Meanwhile, State Department spokeswoman Heather Nauert said Trump’s reply to North Korean leader Kim’s latest letter was hand delivered to Ri in Singapore by Sung Kim, the U.S. Ambassador to the Philippines who has been leading logistical negotiations with the North on its pledge to denuclearize. The White House said earlier in the week that Kim had sent a new letter to Trump and that the president had written a response.

At the United Nations, a summary of the new report by experts monitoring U.N. sanctions against North Korea was sent to the Security Council on Friday. It found that North Korea was continuing with both its nuclear and missile programs and that it was violating sanctions through oil and coal transfers and was flouting an arms embargo and financial sanctions.

Reacting to the report, U.S. Ambassador Nikki Haley said late Friday that reports of Russia violating the Security Council’s resolutions on North Korean laborers were “deeply troubling.”

“Talk is cheap — Russia cannot support sanctions with their words in the Security Council only to violate them with their actions,” she said in a statement posted on Twitter.