AMERICAN spy satellites have detected unusual activity at a suspected North Korean nuclear test site, suggesting that Pyongyang is preparing to carry out its threat to explode an underground atomic device.

A US official said: “We have seen some activity in the area — personnel, vehicles, materials, things of that nature.”

The movements were spotted recently at one of a number of suspected sites that are closely monitored by the Americans. The official insisted that the findings were not conclusive.

The Pentagon assessment was expected to heighten tension in the region, where China, Japan and South Korea all appealed to the regime in Pyongyang not to carry out its threat to detonate an underground nuclear device.

Last night Christopher Hill, the US Assistant Secretary of State, said that Washington had warned North Korea not to conduct the test. “We are not going to live with a nuclear North Korea,” he said.

China, which has close ties with Pyongyang, urged the regime of Kim Jong Il to “keep calm and restrained on the nuclear test issue”. The leaders of China, Japan and South Korea will hold a series of meetings over the weekend to co-ordinate their position.

One option being discussed last night at the United Nations would be a Security Council resolution that warns North Korea that it could face further international isolation. While Japan is pressing for the move, China and South Korea favour persuading North Korea to return to six-party talks it broke off last year.

Beyond concern about the immediate impact of a nuclear test on the region are the longer-term consequences on North Asia, which many fear could undergo a dramatic military escalation.

Yu Myung Hwan, the South Korean Deputy Foreign Minister, gave warning yesterday that a nuclear test by North Korea could “provide a pretext for Japan’s nuclear armament”.

“This will prompt countermoves by China or Russia and lead to a change in the balance of power in Northeast Asia,” he said. The new Japanese Government of Shinzo Abe has pledged to boost the country’s military capabilities, and senior figures in Japan are talking openly about the possibilities of acquiring a nuclear deterrent.

Last month Yasuhiro Nakasone, the former Japanese Prime Minister, who now heads the Institute for International Policy Studies, said that Japan should consider building a nuclear bomb.

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