Defense Secretary Leon Panetta said he's seen no outward sign that North Korea will follow through soon on its plan to conduct a test - following its underground atomic explosions in 2006 and 2009.
But that doesn't mean preparations aren't 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.
North Korea's National Defense Commission said Thursday a nuclear test was part of "upcoming" action directed against the U.S. 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 United States. The North has previously said its launches are for a peaceful space program.
Pyongyang's statement came two days after the U.N. Security Council condemned its December launch of a satellite atop a long-range rocket for violating a ban on ballistic missile activity. The council, with the support of the North's only major ally, China, also tightened sanctions.
"North Korea's statement is needlessly provocative and a test would be a significant violation of United Nations Security Council resolutions," White House spokesman Carney told reporters. "Further provocation 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 the council decision to tighten sanctions would impede the growth of weapons of mass destruction programs in North Korea. He said the U.S. would be taking additional steps. The government later designated as "trade" several entities said to be involved in missile development.
Carney noted the council's warning to take further action in the event of a further launch or nuclear test.
"We judge North Korea by its actions, and provocations like these are significant violations and we act accordingly," Carney said.
North Korea claims the right to build nuclear weapons as a defense against the United States, its foe from the 1950-53 Korean War. The U.S. still maintains 28,000 troops in South Korea as a deterrent against aggression from the North.
North Korea's recently launched rocket has the potential to hit the west coast of the U.S. mainland, but experts say it still doesn't have the capability to make a missile re-enter the atmosphere and hit a target. Nor is it believed to have miniaturized a nuclear device to mount on a missile. A nuclear test could move it closer toward that goal.
The elevation a year ago of Kim Jong Un following the death of his father Kim Jong Il had fueled hope of improved relations with Washington, particularly after the North accepted a substantial offer of food aid in exchange for nuclear concessions. But that agreement collapsed last April when the North conducted a long-range rocket launch.
Carney said the U.S. has not seen a noticeable change in North Korea's behavior.
The Treasury Department levied sanctions Thursday against a Hong Kong-based trading company and two officials of a North Korean bank. The administrative move prohibits them from transactions with Americans and freezes any assets they may have under U.S. jurisdiction.
Treasury described the Tanchon Commercial Bank as the financial arm of Pyongyang's premier arms dealer and main exporter of goods and equipment related to ballistic missiles and conventional weapons.
The bank has also been involved in ballistic missile transactions between the arms dealer, Korea's Mining Development Trading Corporation, and Iran's Shahid Hemmat Industrial Group, a U.S.- and UN-designated organization responsible for developing liquid-fueled ballistic missiles, Treasury said.
"By continuing to expose these entities, and the individuals who assist them, we degrade North Korea's ability to use the international financial system for its illicit purposes," David S. Cohen, the Treasury's undersecretary for terrorism and financial intelligence, said in a statement.
The State Department also designated the Korean Committee for Space Technology, which it said has orchestrated rocket launches and has contributed to development of long-range ballistic missile development.
Associated Press writers Pauline Jelinek and Darlene Superville contributed to this report.