Aerial photographs showed an unusual pattern of activity that indicated possible nuclear preparations after Pyongyang denounced UN sanctions against it as an act of war. The White House said that it would not be surprised if Kim Jong Il defied world opinion again, particularly because the atomic explosion last week was relatively weak.

“The first test, while nuclear, did have a low yield, and perhaps it would not be unreasonable to expect that the North Koreans might want to try something again, to be provocative,” Tony Snow, President Bush’s spokesman, said.

For the first test North Korea may have used a sophisticated plutonium-fuelled device that did not achieve its maximum potential, Western intelligence sources said.

A close examination of the radioactive isotopes scooped up by the Constant Phoenix radiation-monitoring aircraft in the region provided final proof that North Korea had carried out a nuclear test, as it claimed last week.

Experts have been studying the air samples collected by the Constant Phoenix on October 11. That was two days after the underground explosion at a North Korean facility near Punggye, about 60 miles (95km) south of the border with China.

Based on the radioactive isotopes and the seismograph recordings, which registered 4.2 on the Richter scale, US and British experts assessed the device at a yield of less than one kiloton — 1,000 tonnes of TNT, or about 1/20th the size of the Hiroshima bomb.

Intelligence sources said that a kiloton device was “pretty small”, and wondered why North Korea had made such a public pronouncement about its successful achievement. They said it was likely that the explosion was smaller than intended. There have been reports that North Korea warned China before the October 9 test that it was planning a four-kiloton explosion.

Siegfried Hecker, the former head of the Los Alamos National Laboratory, the atomic weapons research facility, told The New York Times that North Korea may have had trouble imploding the device properly. Pyongyang is believed to have enough plutonium for up to ten bombs.

North Korea maintained its bellicose stance yesterday, describing the UN sanctions imposed on it for last week’s test as “a declaration of war”.

In a statement carried by the official Korean Central News Agency, the Foreign Ministry said that if anyone used the UN resolution to infringe on North Korea’s sovereignty “it will deal merciless blows at him through strong actions”. Washington sidestepped such rhetoric. Mr Snow said that North Korea had not declared war, but a second nuclear test “would not be a good thing”. If the North Korean leaders thought that they would escape unpunished “ they’re going to find out that they’re wrong”, he said.

Although Pyongyang’s response was dismissed by the US and South Korea as “nothing new”, the North Korean Foreign Ministry said that the country wanted peace “but is not afraid of war”.

The sanctions, approved by the UN Security Council on Saturday, include a ban on the sale of major arms to North Korea and an inspection of all cargo going in and out of the country.

Christopher Hill, the US Assistant Secretary of State and the chief American nuclear envoy, maintained that the latest North Korean statement was “not very helpful”. He said: “We would all regard a second test as a belligerent answer on North Korea’s part to the international community.”

North Korea, ever defiant, staged a large pro-government rally in Pyongyang yesterday, just as Condoleezza Rice, the US Secretary of State, was embarking on a trip to Tokyo, Seoul, Beijing and Moscow. She plans to discuss the implementation of the United Nations sanctions. Her biggest challenge will be to persuade China to co-operate.

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