The UN Security Council is considering severe economic sanctions against North Korea, after the secretive communist state claimed to have successfully tested a nuclear weapon.

"Once again, North Korea has defied the will of the international community and the international community will respond," U.S. President George Bush told reporters at the White House on Monday.

He added the United States was still attempting to confirm if a nuclear test had actually taken place, but that "such a claim itself constitutes a threat to international peace and security."

At an emergency meeting in New York Monday, members of the Security Council condemned North Korea and demanded that the communist state return to six-party talks on its weapons program.

John Bolton, U.S. Ambassador to the UN, said every Security Council member opposed the test. He asked the members to back a resolution to impose new sanctions against North Korea, to prevent the "abuse of the international financial system."

The proposed sanctions include:

• International inspection of cargo coming to and from North Korea, to limit the proliferation nuclear, chemical and biological weapons;
• Banning any goods -- both luxury and military -- that could be used to manufacture weapons of mass destruction.

Late Monday, Japan added more sanctions to the proposed resolution: banning North Korean ships from entering any Council Member port, and travel restrictions for top North Korean officials.

If the resolution is drafted under Chapter 7 of the UN Charter, it could also authorize the Security Council to take military action against North Korea.

Meanwhile in Canada, Prime Minister Stephen Harper called the test an "irresponsible and dangerous act," while Foreign Affairs Minister Peter MacKay described the situation as "extremely serious."

Russian President Vladimir Putin also unconditionally condemned the test.

CTV's Steve Chao, reporting from Seoul, said there was a "great deal of fear" in South Korea about what the reported test could mean for the future stability of the region.

"There is a great deal of disbelief that North Korea wasn't bluffing, something that experts have been warning about for months now," Chao told CTV Newsnet Monday.

"There is also a great deal of anger at the international community for not doing enough to reign in North Korea."

South Korea put its troops on heightened alert after the North's announcement, which came just minutes before Japanese Prime Minister Shinzo Abe landed in Seoul for a visit.

Defiant

North Korea's ambassador to the UN remained defiant Monday, saying the Security Council should congratulate North Korea for its nuclear test instead of passing "useless" resolutions or statements.

The North said it conducted the underground nuclear test on Monday -- defying an earlier warning from the UN Security Council and opening its crippled economy to the risk of fresh sanctions.

"The nuclear test is a historic event that brought happiness to our military and people," said a quote carried Monday by the Korean Central News Agency, the communist state's official agency.

"The nuclear test will contribute to maintaining peace and stability in the Korean peninsula and surrounding region."

The agency said there had been no radioactive leakage from the test site and that it was carried out with "with indigenous wisdom and technology 100 per cent."

The U.S. Geological Survey said it had detected a 4.2 magnitude tremor in North Korea at 10:35 a.m. local time (8:35 p.m. ET Sunday).

The Japan Meteorological Agency said its data showed a tremor took place around Gilju, on the peninsula's northeast coast around 110 km from the Chinese border.

The size of the bomb is uncertain. South Korean reports put it as low as 550 tons of destructive power but Russia said it was between five and 15 kilotons. The Hiroshima bomb of 1945 was 12.5-15 kilotons.

Non-Proliferation Treaty

Pyongyang pulled out of the nuclear Non-Proliferation Treaty in 2003 and has refused for a year to attend talks aimed at ending its nuclear ambitions.

The Communist state has been threatening such a test as part of its deterrent against what it claims is a possible U.S. invasion.

The UN Security Council imposed an embargo on the import and export of missile-related materials in July, after North Korea test-fired several missiles.

In a statement Monday, the International Atomic Energy Agency said North Korea's reported nuclear test "threatens the nuclear non-proliferation regime and creates serious security challenges."

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.

If the test claim is true, North Korea would become the ninth member of the world's nuclear weapons club.

The others are the United States, Russia, France, Britain, China, India, Pakistan and Israel, which hasn't officially admitted it has a nuclear arsenal.

With files from The Associated Press