Seoul owned up to the experiment last week

The US State Department said the experiment should not have occurred, but praised Seoul for owning up to it.

Japan said the incident was regrettable but that it did not believe South Korea planned to build a bomb.

The admission has embarrassed Seoul and the US, who are pressing North Korea to end its nuclear ambitions.

South Korea says it informed the International Atomic Energy Agency (IAEA), the UN's nuclear watchdog, about the experiment last week.

Officials said a very small amount of uranium - 0.2 of a gram - was produced, by scientists who did not have government approval. Officials also insisted the experiment was conducted for its civilian nuclear power industry, and not for military purposes.

But diplomats and foreign analysts said the uranium produced was so enriched it was almost weapons grade, and far more enriched than is used in civilian programmes.

South Korea has never had, and does not have, enrichment or nuclear reprocessing programs, let alone a weaponization program Oh Joon, South Korea's Foreign Ministry

The technique involved, using high-powered lasers, is also extremely expensive, raising doubts about how it could have gone ahead without official approval.

Disarmament talks

The BBC's correspondent in Seoul, Charles Scanlon, says key questions remain about the quality of the uranium that was produced and when the machinery was destroyed.

On Friday, a foreign ministry spokesman said the machinery was dismantled several months after the experiment, but refused to give a date. Officials had earlier said that the facilities were destroyed immediately afterwards.

South Korea's disclosure may also complicate efforts to persuade North Korea to end its nuclear plans.

The US, Japan and South Korea have been participating in six-party talks aimed at dismantling North Korea's nuclear programme, which are due to resume by the end of the month.

Japan said the revelations were unlikely to derail the talks with North Korea.

Foreign minister Yoriko Kawaguchi said the issue was solely between South Korea and the UN's International Atomic Energy Agency (IAEA).

US state department spokesman Richard Boucher said the scale of South Korea's enrichment activity was "much, much smaller than that being discussed in the situations of North Korea or Iran".

But our correspondent says the North is likely to claim double standards and could delay negotiations.

Some analysts say the international community needs to now come down hard on South Korea to show the rest of the world that it is even-handed on nuclear proliferation.

South Korean officials said the government only found out about the experiments in the last few months, and were now investigating the scientists responsible.

South Korea abandoned a nuclear weapons programme in the 1970s, under intense pressure from the US.