Satellite images indicate North Korea appears to be getting ready for a second nuclear test, officials said Tuesday, as the defiant communist regime held huge rallies and proclaimed that U.N. sanctions amount to a declaration of war.

China, the North's longtime ally and biggest trading partner, warned Pyongyang not to aggravate tensions. The U.N. has condemned the Oct. 9 atomic blast, and U.S. nuclear envoy Christopher Hill told reporters in Seoul on Tuesday that another nuclear explosion would be "a very belligerent answer" to the world.

As the White House acknowledged that the isolated nation might try a second test, Secretary of State Condoleezza Rice launched a diplomatic drive to persuade Asian allies and Russia to intensify North Korea's isolation by enforcing sanctions that the U.N. Security Council unanimously approved on Saturday.

Alexander Vershbow, the U.S. ambassador to South Korea, said Wednesday in Seoul that Rice plans to ask the South to expand its role in a U.S.-led international program to stop the spread of weapons of mass destruction.

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Concern over a second test stems partly from new satellite imagery showing increased activity around at least two other North Korean sites, a senior U.S. defense official said.

The activity, started a number of days ago, included ground preparation at one site and construction of some buildings and other structures, said the official, who spoke on condition of anonymity because it involved intelligence gathering. He said that although the purpose of the structures is unclear, officials are concerned because North Korea has left open the possibility of another test.

A senior South Korean official told foreign journalists that despite signs of a possible second test, 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.

In North Korea, the nation marked the 80th anniversary of the "Down-with-Imperialism Union" — a political platform on which the ruling party was built. North Koreans held parades across the country along with an enormous gathering at a central square in Pyongyang.

In the capital, hundreds of women in brightly colored costumes sang and held bunches of flowers, including some named for Kim Il Sung, the country's first leader and the late father of current leader Kim Jong Il.

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The regime slammed the U.N. measures with a stream of bellicosity in a Foreign Ministry statement released on the official Korean Central News Agency.

"The resolution cannot be construed otherwise than a declaration of a war" against the North, the statement said.

The North also said it "wants peace but is not afraid of war," and that it would "deal merciless blows" against anyone who violates its sovereignty.

It said it wouldn't cave in to "the pressure and threat of someone at this time when it has become a nuclear weapons state."

South Korean nuclear envoy Chun Yung-woo said the North's reaction wasn't surprising, and was full of "the usual rhetoric."

China warned Pyongyang against aggravating tensions.

"We hope North Korea will adopt a responsible attitude ... and come back to resolving the issue through dialogue and consultation instead of taking any actions that may further escalate or worsen the situation," Chinese Foreign Ministry spokesman Liu Jianchao said at a press briefing in Beijing.

But now that the U.S. has confirmed that last week's blast was indeed nuclear, North Korea can be expected to use its new position as a confirmed atomic state to press for direct talks and concessions with Washington — as it did in March 2005, a month after Pyongyang first asserted it had a nuclear weapon.

Hill, the U.S. envoy, said the North was falsely assuming it would win more respect with atomic explosions.

"The fact of the matter is that nuclear tests make us respect them less," he said, adding that the North's comments about sanctions were "not very helpful."

U.S. intelligence has concluded that the North Korean device likely used plutonium, as opposed to uranium, in the Oct. 9 test.

Under a 1994 deal with the U.S. during the Clinton administration, North Korea pledged to freeze its nuclear program, then believed to be based on producing weapons-grade plutonium. But the agreement broke down by 2002 under the Bush administration after revelations of a covert effort by the North to produce highly enriched uranium. Pyongyang soon removed 8,000 spent fuel rods that the International Atomic Energy Agency was monitoring and began to reprocess them into weapons-grade nuclear fuel.

The White House said Tuesday that it wouldn't be surprising if North Korea were to try another nuclear test "to be provocative."

"It would not be a good thing for them, but it certainly would not be out of character," said White House press secretary Tony Snow. "We're not going to discuss any particular matters of intelligence, but if you take a look at the record, I think it is reasonable to expect that the government of North Korea will do what it can to test the will, the determination and the unity of the United Nations."

Asked why it would not be a good thing for North Korea, Snow said, "If they do believe that somehow people are going to give them a pass on this, they're going to find out that they're wrong."

While U.S. officials insist they aren't about to invade, they have taken other steps against North Korea — even before the U.N. resolution — including severing it from the international financial system. That move is believed to have angered the elites that keep Kim in power, and Kim may fear being ousted or possibly even killed.

The North has consistently pressed for direct talks with the U.S. on the financial sanctions and has refused to attend six-nation disarmament talks until the sanctions are lifted. Along with the U.S., the talks include China, Japan, Russia and South Korea.

Now, the North has a new arrow in its quiver: being a confirmed nuclear power recognized as such by the very country whose attention it so desperately craves.

The Bush administration, wary of rewarding the regime's behavior, has consistently refused to talk directly to the North, insisting the issue is a regional concern and seeking to enlist other countries.

On Tuesday, Rice left for Japan, first stop on a four-nation trip, amid clear signs of unease in China and South Korea about even the softened sanctions.

China contends that interdicting ships, although permitted in the U.N. resolution, might needlessly provoke the North and discourage it from returning to the six-nation talks. South Koreans worry about a conventional attack by their unpredictable neighbor.

"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, according to Yonhap news agency. "There should never be a way that causes armed clashes."

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