United Nations sanctions aimed at punishing North Korea for a nuclear test explosion last week amount to "a declaration of war," the Communist country said Tuesday.

The North Korean Foreign Ministry threatened a serious response against any person that uses the UN resolution to impinge upon its sovereignty, saying it "will deal merciless blows at him through strong actions," according to the official Korean Central News Agency.

It was North Korea's first official statement since the security council voted to impose the sanctions, and comes a day after U.S. officials confirmed that radioactive materials found in air samples show that North Korea conducted an underground nuclear explosion on Oct. 9.

The news agency said Pyongyang would like "peace but is not afraid of war," and the UN Security Council "resolution cannot be construed otherwise than a declaration of a war."

North Korea's declaration comes amid growing concern that the reclusive Communist country could conduct a second nuclear test. South Korea and Japan have both warned that Pyongyang could be preparing to explode another nuclear device.

Officials in Washington said satellite imagery had shown increased activity at two potential test sites in North Korea. A second test would not be "out of character" for Pyongyang, said White House spokesman Tony Snow.

Meanwhile, the United States is intensifying its diplomacy to marshal opposition to Pyongyang's nuclear ambitions. Secretary of State Condoleeza Rice has left for Japan, South Korea and China, and one of her deputies, Christopher Hill, is already in the region.

Speaking after meetings with South Korean and Russian officials in Seoul, Hill said North Korea had a fundamental misunderstanding about the resolve of the international community.

North Korea, he said, "is under the impression that once they make more nuclear tests, we will respect them more. The fact of the matter is that … we'll respect them less."

The UN Security Council approved a resolution on Saturday that imposes a ban on the sale of major weapons to North Korea, orders the inspection of cargo entering and leaving the country, and calls for a freezing of assets of businesses that supply Pyongyang's nuclear and ballistic weapons programs.

China has long been one of North Korea's few allies, and officials say Beijing's growing unease over the nuclear and missile testing by Pyongyang is key to a diplomatic resolution of the crisis.

Chinese Foreign Ministry spokesman Liu Jianchao said North Korea should avoid aggravating tensions and rejoin the six-country talks process that it abandoned last year.

With files from the Associated Press