North Korea has a long way to go to dismantle its nuclear weapons program, but its offer to shut down the Yongbyon facility is a "significant" step, a former US intelligence official said Friday.

Andrew Kim, who retired in December as head of the Central Intelligence Agency's Korea Mission Center, said North Koreans had complained to him that they weren't getting due credit for the steps they took to honor their denuclearization commitment after the first US-North Korea summit in June.

"I personally heard from North Korean officials, and they claimed that their concessions are much more valuable and reciprocal action than the US has taken so far," Kim said during a talk at Stanford University. "They say it is as part of (their) commitment to build trust with the US and denuclearization."

US President Donald Trump and North Korean leader Kim Jong-un agreed at their first summit in Singapore to pursue complete denuclearization of the Korean Peninsula in exchange for US security guarantees.

To flesh out the deal, the two leaders are set to meet again in Hanoi, Vietnam, on Feb. 27 and 28.

"I personally believe that North Korea still has a way to go and needs to further demonstrate (their sincerity about) dismantling the key strategic weapons and production infrastructure," the former CIA official said.

As a key member of the US team handling preparations for the Singapore summit, Kim accompanied US Secretary of State Mike Pompeo on all four of his trips to Pyongyang last year.

During that time, North Korea halted its nuclear weapons and ballistic missile tests, destroyed its nuclear testing site in Punggye-ri and dismantled parts of a missile engine facility.

"I know that there was a meaningful kind of step they took. But as much as credit they want, I think they have to do a little more," Kim said.

On one offer, however, the former spy found reason to be hopeful.

"I remember they once again tabled Yongbyon nuclear facility as a topic for negotiation, which I believe is significant," he said, recalling the fourth trip in October. "The US government assesses (that if) the Yongbyon nuclear research facilities shut down, (it would) significantly reduce their capability to produce nuclear weapons. There's even a certain percentage, but it is significant."

North Korea has said that the Yongbyon shutdown is conditional on "corresponding measures" from the United States. (Yonhap)