SEOUL, South Korea – 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 in the wake of U.N. condemnation of its Oct. 9 atomic blast. And U.S. nuclear envoy Christopher Hill told reporters in Seoul 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 approved by the U.N. Security Council.

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.

Japan’s government also has “information” about another possible blast, Foreign Minister Taro Aso told reporters, without elaborating.

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 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.

It was the central government’s first reaction to the sanctions since they were unanimously passed by the Security Council on Saturday.

“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.