U.S. Secretary of State Mike Pompeo warned Russia, China and other nations Saturday against any violation of international sanctions on North Korea.

Pompeo said Washington would take seriously any action that reduces pressure on the North to abandon its nuclear weapons.

The secretary of state said the U.S. has new, credible reports that Russia has violated U.N. sanctions by allowing joint ventures with North Korean companies and issuing new permits for North Korean guest workers.

Pompeo said the reports will be raised with Moscow and "roundly condemned," adding that he remains optimistic that North Korean leader Kim Jong Un will follow through on his pledge to President Trump to denuclearize.

His comments come as a United Nations report accused North Korea of continuing to develop its nuclear and missile programs.

"(North Korea) has not stopped its nuclear and missile programs and continued to defy Security Council resolutions through a massive increase in illicit ship-to-ship transfers of petroleum products, as well as through transfers of coal at sea during 2018," experts wrote in the report seen by Reuters.

Pompeo's remarks at a Southeast Asian regional forum in Singapore drew a swift response from North Korea's foreign minister, who shot back that recent moves by the U.S. have been "alarming."

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The U.S. "is raising its voice louder for maintaining the sanctions against" North Korea, Foreign Minister Ri Yong Ho said, soon after Pompeo's statements.

Ri said that while North Korea has "initiated goodwill measures" including a "moratorium on the nuclear test and rocket launch test and dismantling of nuclear test ground," the U.S. has gone "back to the old, far from its leader's intention."

But Pompeo earlier defended the U.S. position on denuclearizing the Korean Peninsula.

“From my meetings here, the world is united in seeing this achieved,” Pompeo said, according to the Washington Post. “There has not been single country that hasn’t thanked the United States for its efforts in moving the world toward the possibility of achieving this. … I’m optimistic that we will get this done in the timeline, and the world will celebrate what the U.N. Security Council has demanded.”

But he said the timeline for the North's full and final denuclearization remains a work in progress, saying the "the decision is his" and the ultimate timeline will "at least in part" be set by North Korean leader Kim Jong Un, the Post reported.

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Trump had also replied to North Korean leader Kim Jong Un's latest letter in a written response that was hand-delivered to Ri Yong Ho at the Singapore security forum on Saturday.

Heather Nauert, the U.S. State Department spokeswoman, said Trump's reply to Kim was given to Ri by Sung Kim, the U.S. Ambassador to the Philippines who has been leading logistical negotiations with the North on its pledge to denuclearize.

Nauert would not address the content of Trump's letter to Kim.

The Associated Press contributed to this report.