North Korea could be almost ready to carry out its threat to conduct a nuclear test, a U.S. research institute said, pointing to recent satellite photos.

The images of the Punggye-ri site where nuclear tests were conducted in 2006 and 2009 reveal that over the past month roads have been kept clear of snow and that North Koreans may have been sealing the tunnel into a mountainside where a nuclear device would be detonated.

But it remains difficult to discern North Korea's true intentions as a test would be conducted underground.

The analysis was provided Friday to The Associated Press by 38 North, the website of U.S.-Korea Institute at Johns Hopkins School of Advanced International Studies. The latest image was taken Wednesday.

North Korea's powerful National Defense Commission declared its plans Thursday after the U.N. Security Council tightened sanctions in response to a December long-range rocket launch. It described it as part of a ``new phase'' of combat with the United States, which retains 28,000 troops in South Korea and which it blames for leading the U.N. bid to punish Pyongyang.

The North said a nuclear test was part of ``upcoming'' action but did not say exactly when or where it would take place.

38 North concludes that the Punggye-ri site, in the country's northeast, ``appears to continue to be at a state of readiness that would allow the North to move forward with a test in a few weeks or less once the leadership in Pyongyang gives the order.''

South Korean media have cited intelligence officials as saying technical preparations appear complete and the North could be ready to test within days of making a decision to do so.

U.S. officials confirmed Friday that the U.S. has seen some trucks moving around the site. One official said the U.S. is not ruling out that the test could happen in the near future.

But the officials cautioned that, as in previous tests, because it would be done underground, the U.S. may not know much before it actually happens. Officials spoke on condition of anonymity because they were not authorized to discuss intelligence matters publicly.

(AP)