The communist country's first official response to the U.N. sanctions came as Japan and South Korea said the communist nation appeared to be readying for a second atomic blast.
U.S. nuclear envoy Christopher Hill referred to unspecified reports from Seoul that the North was preparing for a second nuclear test, saying it would be "a very belligerent answer" to the world.
Hill said the North's latest comments about the sanctions were "not very helpful" and added that Pyongyang 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's government also had "information" about another possible blast, Foreign Minister Taro Aso told reporters, without elaborating.
In Washington, U.S. Secretary of State Condoleezza Rice prepared to leave for Japan, South Korea, China and Russia on a tour to discuss how to enforce the U.N. sanctions against the North, approved Saturday.
North Korea 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, 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."
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, due to the sensitivity of the information.
China, the North's longtime ally and biggest trading partner, warned Pyongyong 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.
The sanctions include a call to inspect cargo on ships sailing to and from North Korea. But 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 has also 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.