(CBS/AP) 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.

U.S. satellites have detected new construction and vehicle activity at the testing grounds that could be preparations for another test, reports CBS News national security correspondent David Martin. But there is not much U.S. officials like negotiator Chris Hill can do except issue warnings.

"I think we would all regard a second test as a very belligerent answer on North Korea's part to the international community," said Hill.

China, the North's longtime ally and biggest trading partner, warned Pyongyang not to aggravate tensions. The United Nations has condemned the Oct. 9 atomic blast.

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.

The activity detected by satellites started a number of days ago, included ground preparation at one site and construction of some buildings and other structures, said an 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.

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 United States 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.

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