SEOUL, South Korea - North Korea said Tuesday it considered U.N. sanctions aimed at punishing the country for its nuclear test "a declaration of war," as Japan and South Korea reported the communist nation might be preparing a second explosion.

The North broke two days of silence about the U.N. resolution adopted after its Oct. 9 nuclear test with a statement on the official state news agency, as China warned Pyongyang against stoking tensions.

"The resolution cannot be construed otherwise than a declaration of a war" against the North, the statement said. North Korea is known officially as the Democratic People's Republic of Korea.

The chief U.S. nuclear envoy, Assistant Secretary of State Christopher Hill, said the North's response was "not very helpful."

"I think there is a fundamental misunderstanding about what the international community feels about its actions," Hill said in Seoul after a meeting with his South Korean and Russian counterparts.

Hill said he could not confirm South Korean and Japanese reports that the North may be preparing another nuclear explosion, but said a second test would force the international community "to respond very clearly."

North Korea "is under the impression that once they make more nuclear tests that somehow we will respect them more," Hill told reporters after a meeting with U.S. and Russian counterparts. "The fact of the matter is that nuclear tests make us respect them less."

In its statement, North Korea said it would not be intimidated.

The communist nation "had remained unfazed in any storm and stress in the past when it had no nuclear weapons," the statement said. "It is quite nonsensical to expect the DPRK to yield to the pressure and threat of someone at this time when it has become a nuclear weapons state."

Chun Yung-woo, South Korea's top nuclear envoy, dismissed the statement as "the usual rhetoric that they have been using at the time of the adoption of the Security Council resolution."

China has long been one of North Korea's few allies, but relations have frayed in recent months by Pyongyang's missile tests and the nuclear explosion last week.

Chinese Foreign Ministry spokesman Liu Jianchao warned Pyongyang against aggravating tensions, saying the North should help resolve the situation "through dialogue and consultation instead of taking any actions that may further escalate or worsen the situation."

The United States pressed on with a round of diplomacy in Asia aimed at finding consensus on how to implement U.N. sanctions on the North. Secretary of State Condoleezza Rice was expected to go to Japan on Wednesday before traveling to South Korea and China.

Hill stressed that the international community should make the North pay a "high price" for its "reckless behavior."

Japanese Foreign Minister Taro Aso said his government had "information" about another possible blast, and a senior South Korean official said there were signs that the North could be preparing a second test — but emphasized that it was unlikely to happen immediately.

"We have yet to confirm any imminent signs of a second nuclear test," the official said on condition of anonymity because of the sensitivity of the information.

China, whose support for the measures is key to whether they will have any effect on neighboring North Korea, has begun examining trucks at the North Korean border to comply with new U.N. sanctions endorsed over the weekend.

South Korea has said it would implement the U.N. sanctions, but also has been cautious about allowing sanctions to shake regional stability. Seoul has also indicated that it has no intention of halting key economic projects with the North, despite concerns that they may help fund the North's nuclear and missile programs.

"Sanctions against North Korea should be done in a way that draws North Korea to the dialogue table," South Korean Prime Minister Han Myung-sook said Tuesday, according to Yonhap news agency. "There should never be a way that causes armed clashes."

In Washington, U.S. National Intelligence Director John Negroponte's office said Monday that air samples gathered last week contain radioactive materials that confirm that North Korea conducted an underground nuclear explosion.

In a short statement posted on its Web site, Negroponte's office also confirmed that the size of the explosion was less than 1 kiloton, a comparatively small nuclear detonation. Each kiloton is equal to the force produced by 1,000 tons of TNT.

It was the first official confirmation from the United States that a nuclear detonation took place, as Pyongyang has claimed.

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Associated Press writers Bo-Mi Lim and William Foreman in Seoul, Audra Ang in Beijing and Kana Inagaki in Tokyo contributed to this report.