Dismantling North Korea's Yongbyon nuclear complex is the "most immediate next step" in the denuclearization process, an American scientist said Thursday, rebutting claims that the plant is too outdated to consider its closure a major concession from the North.

Siegfried Hecker, a Stanford University professor, made the remarks during a seminar in Seoul, stressing that while Yongbyon's dismantlement will not end the North's nuclear program, it will "dramatically" limit its nuclear capability.

The Yongbyon complex is the North's main nuclear facility. The 5-megawatt reactor at the complex has been the main source of weapons-grade plutonium for the North, though some critics have discounted Pyongyang's offer to close the site, saying the facilities there are too old.

"If you watch, you can see they continue new buildings, new dams under river, new cooling capability, they continuously work on this place," he said.

"This place is up and running and for them to shut this whole thing down is a huge step."

The US expert estimates that North Korea has up to 37 nuclear devices, up from an earlier estimate of 35 in April.

He also said the North's highly enriched uranium increased from some 400-650 kilograms to as much as 700 kg during the cited period, though noting that the estimates are "highly uncertain."

Hecker is considered a top expert on North Korea's nuclear program.

He is also known for having had a firsthand look at North Korea's uranium-enrichment facility during his 2010 visit to the Yongbyon complex. (Yonhap)