Muhammad al-Baradai, director of the International Atomic Energy Agency, is in South Korea to speak at an international conference on science and world affairs.
His visit on Sunday follows Seoul's recent admissions that it conducted a plutonium-based nuclear experiment more than 20 years ago and a uranium-enrichment experiment in 2000.
The Vienna-based IAEA has expressed concern that Seoul failed to report the unauthorised experiments. "Any undeclared activities are a matter of serious concern for me," al-Baradai told reporters upon arrival.
"However, as far as I know now, these have been small experiments. We just wanted to make sure these were experiments and that there were nothing more than these experiments ... (and that) these experiments will not be repeated again without being declared to the organisation."
South Korea says the experiments were purely research, but has acknowledged it should have informed the IAEA.
Submitting a report
Al-Baradai said he believed a report on Seoul's nuclear activities would be ready for submission to the IAEA's board of governors by November.
Asked about the possibility of the issue being reported to the UN Security Council, he said such a decision would be "far down the road." "This is something for the board of governors members to decide," he said. "You cannot speculate on the issue before we have a comprehensive report on these experiments."
"You cannot speculate on the issue before we have a comprehensive report on these experiments."
Director of the International Atomic Energy Agency
During his four-day visit, al-Baradai is expected to meet South Korean Foreign Minister Ban Ki-moon, Unification Minister Chung Dong-young, and Prime Minister Lee Hai-chan.
The IAEA has already sent inspectors there twice, and South Korean officials expect several more visits from the agency.
Plutonium and enriched uranium are two key ingredients of nuclear weapons.
The controversy over South Korea's experiments has threatened to further disrupt troubled efforts to persuade North Korea to dismantle its nuclear weapons programmes.