White House national security adviser John Bolton said on Sunday he believed the bulk of North Korea's weapons programs could be dismantled within a year, although some experts say the complete process could take far longer.

Bolton told CBS's "Face the Nation" 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, if there is full cooperation and disclosure from Pyongyang.

"If they have the strategic decision already made to do that and they're cooperative, we can move very quickly," he 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 Pompeo was due to visit North Korea this week but the State Department has not confirmed any travel plans.

Siegfried Hecker, a nuclear scientist and Stanford University professor, visited the heart of North Korea’s nuclear program in Yongbyon in 2010. He predicted it would take around 10 years to dismantle and clean up a substantial part of that site.

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

U.S. 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.

TRUST BUT VERIFY

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 U.S. President Donald Trump on June 12 gave no details on how or when Pyongyang might surrender its nuclear weapons.

U.S. 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 U.S. officials as saying on Friday.

The Washington Post reported on Saturday that U.S. intelligence officials have concluded that North Korea does not intend to fully give up its nuclear arsenal and is considering ways to hide the number of weapons it has. It also reported Pyongyang has secret production facilities, according to the latest evidence they have.

Bolton refused to comment on intelligence matters but the United States was going into nuclear negotiations aware of Pyongyang's failure to live up to its promises in the past.

"We know exactly what the risks are - them using negotiations to drag out the length of time they have to continue their nuclear, chemical, biological weapons programs and ballistic missiles," he said.

"There's not any starry-eyed feeling among the group doing this," he said. "We're well aware of what the North Koreans have done in the past."

Republican Senator Susan Collins said the report was troubling. "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." Collins stressed the need for "verifiable, unimpeded, reliable inspections" of the North's weapons programs.

Another of Trump's fellow Republicans in the U.S. Senate, Lindsey Graham, echoed the need for skepticism, saying on NBC's "Meet the Press" program, "If it is true that they are saying one thing and doing another, nobody should be surprised."