The U.S. vowed not to be intimidated by North Korean threats Tuesday after Pyongyang reportedly said it could fire a nuclear warhead if Washington did not act to resolve the standoff.

"This is the way North Korea typically negotiates by threat and intimidation," John Bolton, U.S. ambassador to the U.N., said. "It's worked for them before. It won't work for them now."

Meanwhile, the White House said the world may never be able to confirm North Korea's claim that it had successfully tested a nuclear weapon on Monday. White House press secretary Tony Snow suggested that the test may not have been nuclear in nature.

"You could have something that is very old and off-the-shelf here, as well, in which case they've dusted off something that is old and dormant," he said. Snow said the intelligence community would continue to assess the explosion.

The latest North Korean threat came in a report by the country's Yonhap news agency. "We hope the situation will be resolved before an unfortunate incident of us firing a nuclear missile comes," it quoted an unidentified North Korean official as saying. "That depends on how the U.S. will act."

The official said the nuclear test was "an expression of our intention to face the United States across the negotiating table."

Beijing's patience with its longtime ally North Korea appeared to be wearing thin as China's U.N. ambassador suggested that some form of Security Council sanctions against Pyongyang are in order.

"I think there has to be some punitive actions, but also I think these actions have to be appropriate," Ambassador Wang Guangya said.

A permanent U.N. Security Council member, China has a decisive say over how stern a punishment the international community can mete out to North Korea.

China's Foreign Ministry also warned Pyongyang that its staging of a nuclear test would harm the two countries' relations.

CountryWatch: North Korea | South Korea

"The nuclear test will undoubtedly exert a negative impact on our relations," Chinese Foreign Ministry spokesman Liu Jianchao told reporters.

But Liu added that China is "firmly against" a military strike against the North.

"Taking military action against North Korea would be unimaginable," he said.

A South Korean envoy, returning to Seoul from Beijing, said Tuesday that China appeared to be leaning toward backing strong U.N. measures.

Alexander Vershbow, the U.S. ambassador to South Korea, told reporters that the nuclear test would make the possibility of direct talks between Washington and Pyongyang more difficult, Yonhap reported.

The U.N. Security Council was weighing a U.S. proposal for potentially crippling sanctions. America has asked the council to adopt a measure that would aim to curb the North's nuclear and ballistic missile programs, prohibit all trade in military and luxury goods, and crack down on illegal financial dealings.

There have been worries that the reported nuclear test would prompt Japan to build its own bomb. But Japanese Prime Minister Shinzo Abe told lawmakers Japan's anti-nuclear policy would remain unchanged.

"There will be no change in our non-nuclear arms principles," Abe said.

Japan's pacifist constitution bars the use of force to settle international disputes, and Japan has maintained a policy of not producing, possessing or using nuclear weapons.

South Korea said that it believed the North had exploded a nuclear device on Monday, but officials claimed that it might take up to two weeks to confirm whether the test was successful.

Although the reported test drew worldwide condemnation and talk of harsh sanctions, the South said it would stick with its efforts to engage the North, though the policy would be reviewed.

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North Korea celebrated a holiday Tuesday marking the 61st anniversary of the Workers' Party of Korea. There was no traffic across a key bridge on a border river between China and North Korea.

China canceled leave for its soldiers along the North Korean border and some units were conducting anti-chemical weapons drills, the pro-Beijing Wen Wei Po reported in Hong Kong. The paper didn't elaborate.

There was no sign of heightened security in the Chinese border city of Dandung, and reporters saw two boatloads of North Korean tourists on the river, smiling and waving to people on the Chinese shore.

The Associated Press contributed to this report.