Seoul acknowledged on Wednesday that a South Korean firm had nearly sold Iran a substance that can be used to boost a nuclear explosion but denied an exile group's allegation that the sale had been completed.
The National Council of Resistance of Iran (NCRI), listed by the US State Department as a terrorist organisation, said Tehran had used front companies to obtain the substance, a hydrogen isotope called tritium, which also has civilian uses.
Ali Safavi, an official of the NCRI, which has reported accurately on Iran's atomic programme in the past, told a news conference in Brussels on August 25 that Iran had successfully smuggled tritium from South Korea.
However, Du-Ock Beck, director of the Export Control Licensing and Enforcement Division at South Korea's Ministry of Commerce, Industry and Energy, said the sale never went through.
"The government believes such misunderstanding derives from the fact that on December 3 last year a Korean company planned to ship tritium to Iran after initially acquiring the export-controlled but highly versatile substance from France," he said.
Tritium is used in luminous paint and in testing the safety of drugs but can be combined with another hydrogen isotope, deuterium, to act as a booster in atomic bombs.
Beck said the South Korean company had decided not to go ahead with the deal when France asked it to submit documentation on the material's end-user on December 24, 2004.
"After conducting an investigation...the Korean government found that no Korean company exported to Iran the substance in question, tritium," Beck said in an e-mail to Reuters.
"The Korean government hereby issues a notification that Ali Safavi's alleged statement through Reuters that a Korean company transferred the substance tritium to Iran is unfounded and untrue," he added.
The Korean customs service confirmed on Tuesday that there was no record of a Korean company shipping tritium, Beck said.
South Korea ranks sixth in output of nuclear power in the world, producing about 40 percent of its electric power at its 20 nuclear plants.