The South Koreans are denying strenuously denying reports they are developing a an illegal uranium enrichment and nuclear weapons programme.
A government official said South Korea's shock declaration yesterday that government scientists enriched uranium four years ago was a by-product of tests to separate materials using a laser.
The official said the revelation was unlikely to upset international efforts to end North Korea's nuclear ambitions.
Mr Oh Joon, director general for international organisations at the South Korean Foreign Ministry, told reporters the enrichment equipment had been dismantled a few months after the single experiment aimed at separating uranium.
"This was an isolated case," he said.
He said the South Korean government scientists' enrichment experiment was fundamentally different from non-proliferation treaty violations by other countries.