The Korea Atomic Energy Research Institute burned a box containing the natural, depleted and enriched uranium after mistakenly sorting it as ordinary industrial waste, the unnamed official told South Korea's Yonhap news agency.

"The uranium was such a small amount that it wasn't to the level that could cause harm to the human body or the environment in the course of the incineration," the official, requesting anonymity, was quoted by Yonhap saying.

The institute, in a statement posted on its website later Thursday, said it believed the box of uranium was mistakenly burnt by a private waste disposal company in May.

The state-run institute, about 180 kilometres (111 miles) south of Seoul, said it thought a leak of harmful radioactive material from the burning had been unlikely, Yonhap said.

The centre's statement said it had recently reported the incident to the science and technology ministry and the UN's nuclear watchdog, the International Atomic Energy Agency.

The box contained 1.9 kilograms (four pounds) of natural uranium, 0.8 grams of depleted uranium and 0.2 grams of enriched uranium, it said.

Reports of the incident come after the world's largest nuclear plant, in Japan, shut down last month after leaking a small amount of radiation following a powerful earthquake.

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