White House national security adviser John Bolton says most of North Korea’s weapons programs could be dismantled within a year.

However, some experts say the complete process could take far longer.

Mr Bolton told the CBS network in the United States that Washington has devised a program to dismantle North Korea’s weapons of mass destruction – chemical, biological and nuclear – and ballistic missile programs in a year, provided there was full cooperation and disclosure from Pyongyang.

“If … they’re cooperative, we can move very quickly,” Mr Bolton said.

“Physically we would be able to dismantle the overwhelming bulk of their programs within a year.”

He said Secretary of State Mike Pompeo will likely discuss that proposal with the North Koreans soon.

The Financial Times reported that Mr Pompeo was due to visit North Korea this week, but the State Department has not confirmed any travel plans.

Some experts disputed Mr Bolton’s optimistic time frame, at least not for the verification of the dismantling, as well.

“It would be physically possible to dismantle the bulk of North Korea’s programs within a year,” said Thomas Countryman, the State Department’s top arms control officer under President Barack Obama.

“I do not believe it would be possible to verify full dismantlement within a year, nor have I yet seen evidence of a firm DPRK decision to undertake full dismantlement.”

North Korea is completing a major expansion of a key missile-manufacturing plant, the Wall Street Journal reported on Sunday, citing researchers who have examined new satellite imagery from San Francisco-based Planet Labs Inc.

Images analysed by the Middlebury Institute of International Studies at Monterey, California shows that North Korea was finishing construction on the exterior of the plant around the time North Korean leader Kim Jong Un met with US President Donald Trump in Singapore in June, the report said.

The Chemical Material Institute in Hamhung makes solid-fuel ballistic missiles, which could allow North Korea to transport and launch a missile more quickly, compared to a liquid-fuel system that requires lengthy preparation.

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.

South Korea media reported on Sunday that US envoy Sung Kim, the American ambassador to the Philippines, met with North Korean officials at the border on Sunday to coordinate an agenda for Mr Pompeo’s next visit to North Korea.

US intelligence is not certain how many nuclear warheads North Korea has. The Defense Intelligence Agency is at the high end with an estimate of about 50, but all the agencies believe Pyongyang is concealing an unknown number, especially smaller tactical ones, in caves and other underground facilities around the country.

North Korea agreed at the summit to “work toward denuclearization of the Korean Peninsula,” but the joint statement signed by North Korean leader Kim Jong Un and US President Donald Trump on June 12 gave no details on how or when Pyongyang might surrender its nuclear weapons.

US intelligence agencies believe North Korea has increased production of fuel for nuclear weapons at multiple secret sites in recent months and may try to hide these while seeking concessions in nuclear talks with the United States, NBC News quoted US officials as saying on Friday.