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next step" in the denuclearization process, an American scientist said Thursday, rebutting claims that the plant is too outdated to consider its closure a major concession from the North.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.