Satellite imagery suggests North Korea may be preparing a second nuclear bomb test, U.S. officials say.

Japanese officials also had "information" about another possible test, Japan's Foreign Minister Taro Aso told reporters, without elaborating.

In response, China has told the government in Pyongyang to not aggravate tensions. U.S. envoy Christopher Hill told reporters in Seoul that a second test would be a "very belligerent answer" to the world.

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

U.S. Secretary of State Condoleezza Rice has warned North Korea that a second nuclear test would be a provocation that would only deepen its isolation. Rice was speaking as she prepared for a tour of Asia to bolster the UN sanctions.

However, a South Korean official said despite the evidence of activity at potential test sites, a second test was unlikely to happen in the short term.

"We have yet to confirm any imminent signs of a second nuclear test," the official said.

The warning comes as North Korea denounced UN sanctions aimed at punishing the country for its nuclear test as a "declaration of war," the communist country's state media said.

The central government in Pyongyang, breaking two days of silence on the sanctions, said North Korea is now a nuclear power and will "deal merciless blows" against any state that violates its sovereignty.

"The DPRK (Democratic People's Republic of Korea) had remained unfazed in any storm and stress in the past when it had no nuclear weapons," state media quoted a foreign ministry spokesperson as saying.

"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."

"The DPRK wants peace but is not afraid of war," the spokesperson added. "It wants dialogue but is always ready for confrontation."

The United States confirmed Monday that North Korea had indeed conducted an underground nuclear explosion on Oct. 9, which brought worldwide condemnation and harsh sanctions against the reclusive state.

Russia, China, the U.S., South Korea and Japan have all been holding talks with Pyongyang to persuade it to abandon its nuclear weapons program.

Russian Defence Minister Sergei Ivanov said he didn't rule out North Korea's capability to carry out a second test, and warned that Moscow's reaction to such a test would be "negative."

China, considered to be crucial to the success of the sanctions which call on nations to inspect all import and export cargo from North Korea, began examining trucks at the North Korean border.

The UN measures call for all countries to inspect cargo to and from North Korea, and ban trade in major weapons and materials that could be used in the North's ballistic missile and weapons of mass destruction programs.

With files from The Associated Press