U.S. officials, on the heels of confirming Monday that North Korea did indeed test a nuclear device, believe Pyongyang is planning more tests, and soon.

Officials told FOX News that the first test, conducted Oct. 9, was "more of a fizzle than a pop," and that the North Koreans feel the need to prove they can successfully detonate a nuclear weapon.

"It would not be unreasonable to assume the North Koreans are planning a second test," White House press secretary Tony Snow said Tuesday.

U.S. nuclear envoy Christopher Hill, meanwhile, called reports of a possible second nuclear test "a very belligerent answer" to the world.

Hill 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 told reporters in Seoul.

Japan also had "information" about another possible blast, Foreign Minister Taro Aso told reporters, without elaborating.

Secretary or State Condoleezza Rice first raised the possibility of more tests on Monday night.

"We're watching it, obviously, and discussing it with other parties, as well," Rice told FOX News. "I think it goes to say that that would further deepen the isolation of North Korea, and I hope they would not take such a provocative act.

Talk of more North Korean tests came as the Pyongyang regime of Kim Jung-Il declared Tuesday that any U.N. sanctions over its nuclear program would be treated as a "declaration of war" and warned of "merciless blows" if its sovereignty was violated.

CountryWatch: North Korea | South Korea

North Korea slammed the U.N. measures 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, also known as the Democratic People's Republic of Korea, the statement said.

The North also warned 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, the North's longtime ally and biggest trading partner, 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 regular press briefing in Beijing.

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The sanctions include a call to inspect cargo on ships sailing to and from North Korea. China and Russia contend that interdicting vessels might needlessly provoke the North.

Liu wouldn't directly comment on why China refuses to board ships but insisted that "the Chinese side has always implemented Security Council measures seriously and in a responsible manner."

"This time is no exception," Liu said.

South Korea has said it would fully comply with the U.N. sanctions resolution. Seoul also has 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, according to Yonhap news agency. "There should never be a way that causes armed clashes."

The Associated Press contributed to this report.