“North Korea’s statement is needlessly provocative, and a test would be a significant violation of United Nations Security Council resolutions,” White House spokesman Jay Carney told a press briefing.

“Further provocations would only increase Pyongyang’s isolation, and its continued focus on its nuclear and missile program is doing nothing to help the North Korean people,” Carney said.

North Korea is preparing for a nuclear test that may target the United States, South Korea’s Yonhap news agency reported on Thursday, citing a statement by the North’s top military body.

"In the new phase of our century-long struggle against the United States, we do not hide the fact that various satellites, long-range missiles that we will continue to launch and a high-level nuclear test we will conduct will target our sworn enemy, the United States," North Korea's National Defense Commission said in a statement published by the state Korean Central News Agency (KCNA).

Pyongyang’s statement evoked a strong reaction from the international community. South Korea expressed deep concern and urged its neighbor to abandon any nuclear test plans.

German Foreign Minister Guido Westerwelle described North Korea’s plans as a provocation that will only further isolate Pyongyang in the world.

He urged Pyongyang to comply with UN Security Council resolutions and to “not aggravate the situation on the Korean Peninsular with irresponsible actions.”

Shortly before the commission issued its declaration, US envoy on North Korea Glyn Davies urged Pyongyang not to explode an atomic device.

"Whether North Korea tests or not, it's up to North Korea. We hope they don't do it. We call on them not to do it. It will be a mistake and a missed opportunity if they were to do it."

In Russia, some MPs dismissed Pyongyang’s statement as a demonstration of force in word only, that will not be followed up by concrete action.

“These latest statements about forthcoming tests are dictated by a wish to secure itself against possible outside interference,” said Leonid Kalashnikov, deputy head of the Foreign Relations Committee in the lower house of the Russian parliament.

Another MP warned of an international backlash if Pyongyang went ahead with the test.

“It is North Korea’s right, but the reaction of the international community will be extremely negative,” said State Duma CIS Committee head Leonid Slutsky.

The North "has completed technical preparations” for its third nuclear test and can conduct it “in a few days" if North Korean leader Kim Jong-un decides, a South Korean intelligence source earlier told Yonhap.