The US has denounced North Korea for threatening a third nuclear test, calling the warning "needlessly provocative".
North Korea, which put a satellite into orbit last month, made the threat on Thursday after an unanimous vote at the UN Security Council that expanded the number of entities on an international blacklist.
Defence Secretary Leon Panetta said he has seen no outward sign that North Korea will follow through soon on its plan to conduct a test. But that does not mean preparations are not under way.
"They have the capability, frankly, to conduct these tests in a way that make it very difficult to determine whether or not they are doing it,'' Panetta told reporters.
He added that the US administration was "prepared to deal with any kind of provocation from the North Koreans".
North Korea's National Defence Commission said a nuclear test was part of "upcoming" action directed against the US, but did not say exactly when or where it would take place. The commission, led by leader Kim Jong-un, also made clear that its long-range rockets are designed to carry warheads aimed at striking the US.
White House spokesman Jay Carney said a nuclear test "would only increase Pyongyang's isolation".
"North Korea's statement is needlessly provocative and a test would be a significant violation of United Nations Security Council resolutions,'' he said.
North Korea insists that its December 12 launch was a peaceful scientific mission. Outside experts agree that Pyongyang succeeded in putting a satellite in orbit, but US officials charged that the launch was aimed at developing a ballistic missile that could reach the US.
The US, supported by Japan and South Korea, spearheaded the new UN resolution. China, North Korea's main ally, supported the bid after lengthy negotiations in which it agreed to expand the number of entities under existing restrictions rather than create a new set of sanctions.
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The United States added names to a blacklist that freezes any US-based assets of designated individuals and groups
and makes it a crime for anyone in the United States to assist them.
"North Korea will continue to face isolation if it refuses to take concrete steps to address the concerns of the international community over its nuclear and missile programmes," the State Department said in a statement.
The State Department blacklisted the Korean Committee for Space Technology, which carries out Pyongyang's rocket launches, and two related individuals.
The Treasury Department imposed sanctions on a Hong Kong-based trading company, Leader (Hong Kong) International Trading Ltd, which it accused of assisting shipments for North Korea's main arms dealer.
It also blacklisted two Beijing-based representatives from the Tanchon Commercial Bank, which it said was the financial arm of Korea's Mining Development Trading Corporation, accused of missile transactions with Iran.
North Korea's sole major ally China urged "all relevant parties" to show restraint .
"All relevant parties should refrain from action that might escalate the situation in the region," foreign ministry spokesman Hong Lei told reporters in Beijing.