US Secretary of State Rex Tillerson says the United States is getting evidence that international sanctions are "really starting to hurt" North Korea.
US President Donald Trump told Reuters that Russia was helping North Korea evade international sanctions and that Pyongyang was getting closer every day to being able to deliver a long-range missile to the United States.
Mr Tillerson told reporters the Russian failure to comply with the UN measures "primarily" concerned fuel "but some other areas potentially as well". He did not provide details.
The Kremlin later described the allegation as absolutely groundless, Interfax quoted a source at the Russian Foreign Ministry as saying.
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Nevertheless, Mr Tillerson said he was confident the pressure would eventually bring North Korea to the negotiating table over its nuclear and missile programs.
"We are getting a lot of evidence that these sanctions are really starting to hurt," Mr Tillerson said, citing intelligence and anecdotal evidence from defectors.
He said Japan told a conference on North Korea in Vancouver on Tuesday that more than 100 North Korean fishing boats had drifted into its waters and two-thirds of those aboard them had died.
"What they learned is that they are being sent out in the winter time because there's food shortages and they are being sent out to fish with inadequate fuel to get back," he said.
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South Korean President Moon Jae-in had attributed North Korea's recent willingness to talk to South Korea to the pain of sanctions, Mr Tillerson told an event at Stanford University in California.
China did not attend the Vancouver meeting, where 20 nations agreed to step up sanctions pressure on the North, but Mr Tillerson highlighted Beijing's role.
"We have never had Chinese support for sanctions like we're getting now," he said. "Russia's a slightly different issue, but the Chinese have leaned in hard on the North Koreans."
He said he was confident the sides would eventually get to the negotiating table and he wanted North Korea to know that, when that happened, the United States had "very, very strong military options standing behind me".