Investigators from the International Atomic Energy Agency have launched a week-long inspection into South Korea's past nuclear experiments involving undeclared fissile material.

Five IAEA inspectors arrived at the Korean Atomic Energy Research Institute, the nation's main nuclear research center, located in Daejeon, some 160 km south of Seoul, Monday morning.

Earlier this month South Korea acknowledged its scientists produced small amounts of plutonium and enriched uranium in two controversial experiments, one in 1982 and the other in 2000, without reporting to the government.

The IAEA officials flew into South Korea on Sunday for the second inspection in less than a month into the controversial laboratory experiments that touched off international suspicions over Seoul's nuclear ambitions.

South Korea has stressed that the one-time experiments were purely academic activities that had nothing to do with nuclear weapons. But critics from abroad have continued to harbor suspicions over Seoul's nuclear intentions.

This week's inspection will last until Saturday. The IAEA officials are scheduled to leave the country on the following day.