By conducting this nuclear test, North Korea has opened its crippled economy to the risk of fresh sanctions.

South Korea?s military ordered the army to step up a state of alert after Pyongyang announced its first-ever nuclear test, which brought unusual criticism from fellow communist China. Pyongyang?s move, which came about 30 minutes before Japanese Prime minister Shinzo Abe landed in Seoul for a visit, could heighten regional tension and could deal a fresh foreign policy blow to US President George W. Bush ahead of mid-term polls. The White House branded the act ?provocative? and said it expected the UN Security Council to take immediate actions.

North Korea?s announcement pushed the dollar to an eight-month high against the yen and helped shove oil above $60 a barrel. In Seoul, the won fell 1.5 percent to two-month lows and the main stock index tumbled as much as 3.6 percent.

The US Geological Survey said it had detected a 4.2 magnitude tremor in North Korea at 10:35 local time (0135 GMT).

In a report from Moscow, a Russian Defence Ministry official was quoted as saying it was ?100 percent sure? that North Korea carried out a test at around that time.

Pyongyang?s official Korean Central News Agency (KCNA) said there was no leak or danger from its test, which measurements by the Japan Meteorological Agency showed took place around Gilju, on the country?s northeast coast and around 110 km (70 miles) from the Chinese border. ?The nuclear test was conducted with indigenous wisdom and technology 100 percent,? KCNA said. ?It marks a historic event as it greatly encouraged and pleased the KPA (Korean People?s Army) and people that have wished to have powerful self-reliant defence capability.?

Analysts say North Korea probably has enough fissile material to make six to eight nuclear bombs but probably lacks the technology to devise one small enough to mount on a missile. Pyongyang did not indicate if there would be further tests. India and Pakistan, the last countries to conduct tests in 1998, carried out several each.

Gary Gibson of Australia?s Seismology Research Centre estimated the blast at about one kiloton. That was dwarfed by India?s biggest ? around 45 kilotons ? and the 10-kiloton bomb dropped on the Japanese city of Hiroshima in 1945.

The UN Security Council urged North Korea last week not to carry out a test, warning of unspecified consequences if it did. Japanese Foreign minister Taro Aso said Tokyo was considering further sanctions on North Korea and might push for a fresh Security Council resolution if the nuclear test were confirmed.

The Security Council was due to meet yesterday to officially nominate South Korean Foreign Minister Ban Ki-moon as next United Nations secretary-general, at which time it was likely to discuss North Korea?s move.

A US official, speaking on condition of anonymity, said the North had given China a 20-minute warning of its test and Beijing had immediately alerted Washington, Tokyo and Seoul.

Although it is the closest reclusive North Korea has to an ally, China described the nuclear test as ?brazen? and called on its neighbour to stop any action that would worsen the situation. ?It will be like America, where everybody thinks he has the right to own a gun. The first country to be encouraged by this will be Iran, and then other countries in the Middle East?, a Chinese analyst says.

Overview of the nuclear seven?s tests since 1945

1. UNITED STATES:

■ First nuclear test: 1945

■ Most recent nuclear test: 1992

■ Total tests: 1,030 (815 underground)

■ The United States has conducted more tests than the rest of the world, and was the first and only country to use a nuclear weapon in wartime. It has signed the Comprehensive Test Ban Treaty, but not yet ratified it.

2. RUSSIA

■ First nuclear test: 1949

■ Most recent nuclear test: 1990

■ Total tests: 715 (496 underground)

■ Russia was the second nation in the world to conduct nuclear tests. In 1961 it exploded what was the world?s largest-ever nuclear device - around 50 megatons, equivalent to 50 million tons of TNT - on the Arctic island of Novaya Zemlya.

3. UNITED KINGDOM

■ First nuclear test: 1952

■ Most recent nuclear test: 1991

■ Total tests: 45 (24 underground)

■ Britain tested its first nuclear weapon on Monte Bello Islands, Australia. Atmospheric tests were carried out there until 1956. Britain has ratified the Comprehensive Test Ban Treaty.

4. FRANCE

■ First nuclear test: 1960

■ Most recent nuclear test: 1996

■ Total tests: 210 (160 underground)

■ France conducted six tests in the South Pacific between 1995 and 1996. Faced with an international outcry, it announced in January 1996 that France would halt testing.

5. CHINA

■ First nuclear test: 1964

■ Most recent nuclear test: 1996

■ Total tests: 43 (22 underground)

■ China?s tests were all conducted at the Lop Nur base in Xinjiang, west China.

6. INDIA

■ First nuclear test: 1974

■ Most recent nuclear test: 1998

■ Total tests: 7

■ In 1966, India declared it could produce nuclear weapons within 18 months. Eight years later it tested a device of up to 15 kilotons and called it a ?peaceful nuclear explosion?. In May 1998, India stunned the world by conducting six underground tests in Pokharan, Rajasthan, and declaring itself a nuclear state.

7. PAKISTAN

■ First nuclear test: 1998

■ Most recent nuclear test: 1998

■ Total tests: 6

■ In 1972, following its third war with India, Pakistan started a secret nuclear weapons programme to match India?s developing capability. It responded to India?s nuclear tests in 1998 by exploding six underground devices in the Baluchistan region near the border with Afghanistan.

• Israel is widely believed to have nuclear weapons but has never admitted it or carried out a public test. South Africa developed nuclear weapons during the apartheid era, but renounced them in 1993-4.