Siegfried Hecker, a nuclear scientist and Stanford University professor, has predicted it would take around 10 years to dismantle and clean up a substantial part of North Korea's Yongbyon nuclear site.
Meanwhile, the Trump administration is expanding the capabilities of U.S. nuclear weapons, asking Congress last month to approve the conversion of some high-yield W76-1 warheads to the smaller W76-2 version.
CNN host Fareed Zakaria paraphrased a famous Ronald Reagan quote to knock President Donald Trump's approach to North Korean dictator Kim Jong Un, as he claimed the president likes to "trust, trust, trust, trust and never verify".
In the weeks following the Trump-Kim meeting, speculation - especially in the US media - has been mounting over whether North Korea truly intends to denuclearize and what exactly the process of denuclearization would look like.
The tweet comes amid concerns surrounding both leaked reports from the intelligence community and commercial satellite imagery from private groups indicating that North Korea may be trying to obscure their nuclear activities and improve some existing nuclear sites.
"It is perfectly rational for North Korea to shift the emphasis to developing solid fuel missiles now that it already has a suite of liquid fuel missiles to deter an attack", he said. Even that was viewed as bullish by nonproliferation experts considering the scale of North Korea's weapons program and its history of evasion and reluctance to allow verification of disarmament agreements.
The Washington Post on Sunday quoted unnamed U.S. officials who were briefed on intelligence findings that point to preparations for deceiving the United States, while NBC news has said Pyongyang is increasing its nuclear fuel production.
"He wants to understand the Russian position, but, more importantly, he wants Putin to understand our position", Bolton said.
He said the one-year program the U.S.is proposing would cover all the North's chemical and biological weapons, nuclear programs and ballistic missiles.
A US official told The Associated Press that the Post's report was accurate and that the assessment reflected the consistent view across USA government agencies.
Bolton also said on "Face the Nation" that during the US-North Korea summit in Singapore, Kim was "very emphatic several times" about turning over the arsenal, which was a change from previous regimes. Independent journalists were not given access to cover the event depicted in this image distributed by the North Korean government.
Secretary of State Mike Pompeo reportedly is preparing for his third trip to North Korea later this week to discuss a proposed timeline for full denuclearization of the Korean peninsula. He also said Trump and Kim should decide whether to move the nuclear weapons out of North Korea to dismantle them or do it inside the country.
"I think the Secretary has addressed that extensively, in which he said that North Korea is very clear in terms of our expectations".
The U.S. statement said that Sung Kim had led a delegation that met with North Korean counterparts in the village of Panmunjom on Sunday, where they discussed the next steps toward implementing the joint declaration signed by Trump and Kim Jong Un last month.