Inspectors from the International Atomic Energy Agency have resumed investigations into South Korea's recently-revealed nuclear experiments.
The South Koreans have shown interest in the nuclear cycle
The IAEA has expressed concerns about the illicit nuclear activities, which involved plutonium and uranium.
Seoul has repeatedly stressed it has no intention of building nuclear weapons.
North Korea has said it will not return to six-nation talks on its own nuclear programme until South Korea's research was "fully probed".
The IAEA team returned to South Korea over the weekend, for the second time this month, to continue investigations into the country's clandestine nuclear experiments.
On Monday a team of five inspectors arrived at the country's main nuclear research centre in the city of Daejon, to begin a week of checks.
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 counter criticism of its own nuclear ambitions.
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.
North Korea also accuses the United States of double standards, for not being more critical of the revelations from South Korea.