The report came as Japan finds itself under pressure to soften its hardline stance against Pyongyang following US President Donald Trump's landmark summit with North Korean leader Kim Jong Un last month.
But the respected 38 North monitoring group said last week that not only were operations continuing at the North's main Yongbyon nuclear site, it was also carrying out infrastructure works, citing recent satellite imagery.
Details were scant, but Bolton said it would see North Korea's nuclear, biological, and chemical arsenals totally wiped out, along with its entire ballistic missile program.
Washington and Pyongyang have yet to negotiate a plan for how North Korea will denuclearize.
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Since then, the young North Korean leader has been on a diplomatic push, meeting with world leaders and brandishing his image as a global statesman.
Secretary of State Mike Pompeo is expected to meet with North Korean officials soon to continue negotiations.
But Bolton also said the U.S. has a plan for Kim to decnuclearize within a year, something Secretary of State Mike Pompeo will discuss when he meets with North Korean officials this week.
North Korea's past behavior offered a cautionary tale for the Trump administration ahead of the Singapore summit, with Pyongyang having reneged on earlier airy promises regarding denuclearization.
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The recent reports, based on U.S. intelligence leaks, suggest the country is still upgrading its nuclear enrichment sites, among other activities.
Asked about the USA diplomat's visit, Kim Eui-keum, a spokesman for South Korea's Blue House, said that they were aware of the meeting but that it would be inappropriate to comment further. They are "only" reports but they are deemed accurate by respected North Korea watchers.
"We have developed a program", Bolton said, "...about really how to dismantle all of their WMD [weapons of mass destruction] and ballistic missile programmes in a year".
Xi, however, apparently tried to temper North Korea's rapid moves to get closer to the United States. "Meeting with Kim Jong Un was an interesting and very positive experience". This year's UNGA will take place from September 18 to September 30.
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National Security Adviser John Bolton said he wouldn't comment on intelligence matters, but said the administration is "very well aware of North Korea's pattern of behavior" while negotiating with the US. "I will just say this: Not answering the specific reports we've seen over the weekend ... we are using the full range of our capabilities to understand what North Korea is doing". But North Korean officials are suggesting that they declare far fewer. "North Korea has a long history of cheating on agreements that it's made with previous administrations", she said on CNN's "State of the Union".