"It appears circumstances are turning in a direction that makes it difficult to be optimistic about whether the six-party talks can be held or not (this month)," Ban Ki-Moon said in a radio interview.

Ban spoke as officials from Japan, South Korea and the United States met in Tokyo to prepare for a new round of talks and a high-ranking Chinese delegation left Beijing for North Korea.

At their last meeting in Beijing in June, the two Koreas, the United States, China, Japan and Russia agreed to meet again before the end of this month.

Since then North Korea has adopted a more belligerent tone toward the United States and South Korea while expressing doubts about the value of attending further discussions.

In recent days Seoul's admission that South Korean scientists carried out unauthorised experiments to enrich uranium and produce plutonium has complicated matters.

On Wednesday North Korea warned that South Korea's enrichment experiment could fuel an arms race in Northeast Asia.

The meeting in Tokyo brought together Mitoji Yabunaka, head of the Japanese foreign ministry's Asian and Oceanian affairs bureau, South Korean Deputy Foreign Minister Lee Soo-Hyuck and US Assistant Secretary of State for East Asia and Pacific affairs James Kelly.

Meanwhile Li Changchun, a member of the Chinese Communist Party's powerful nine-strong standing committee, led a senior delegation from China to North Korea where he is expected to meet leader Kim Jong-Il.

The trip by Li is widely seen as an effort to get Pyongyang's agreement to attend a fourth round of six party talks.

Analysts say the recent revelations about South Korea's nuclear activities may give North Korea an excuse for not doing so.

South Korea insists it has no interest in acquiring nuclear weapons and the experiments were no more than unauthorized lab tests by scientists to satisfy academic curiosity.

Chang In-Soon, president of the Korea Atomic Energy Research Institute, said the plutonium experiment in 1982 extracted a miniscule amount of the nuclear fuel.

"Scientists at that time dissolved 2.5 kilograms of spent fuel in nitric acid and there was seven liters of such liquid. They used two liters of this liquid and extracted both uranium and plutonium," he told Yonhap news agency.

"On assumption that they were able to extract all the plutonium in there without any loss, the maximum amount of plutonium obtained must have not been greater than 86 miligrams.

"It is just ridiculous to fuss about what was just no more than a mere trace of plutonium that appeared in a lab test."

The experiment to enrich uranium in 2000 produced 0.2 grams (0.007 ounces) of nuclear fuel, according to South Korea.

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