By Charles Scanlon
BBC correspondent in Seoul
A team from the International Atomic Energy Agency arrives in South Korea on Sunday to resume an investigation into secret nuclear experiments.
The South Koreans have shown interest in the nuclear cycle
The IAEA has expressed concern about Seoul's nuclear activities over the last two decades.
Seoul has repeatedly stressed it has no intention of building nuclear weapons.
North Korea says it will not return to talks on its own nuclear programme until the US drops its double standards on nuclear proliferation in the region.
The inspectors are returning to South Korea for the second time this month to continue investigations into illicit nuclear experiments.
The director-general of the IAEA, Mohammed el-Baradei, is also due to visit Seoul next month in a further sign of the agency's concern.
South Korea has admitted its scientists conducted tests in 1982 and again four years ago to extract plutonium and to enrich uranium, two separate routes to an atomic bomb.
But the government says the tests were on too small a scale to be significant and has blamed curious scientists acting without official authorisation.
South Korean officials are liaising with the IAEA over the revelations
Many questions remain unanswered.
The inspectors will investigate why South Korea failed to declare three separate sites for the production of uranium metal which was used as a raw material for some of the experiments.
North Korea is using the South's predicament to divert criticism of its own well-advanced atomic bomb programme.
The state news agency has backed up earlier statements that the North will not return to the negotiating table until South Korea's activities have been fully investigated.
It accuses the United States of double standards, for its relatively relaxed public response to the revelations from South Korea.