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

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 to The Associated Press by 38 North, the website of the 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 the plans 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 some trucks have been seen 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.

In 2006, North Korea detonated a nuclear device just six days after it announced its plans to do so, and in 2009, 26 days after the announcement. Both tests came weeks after the U.N. Security Council had condemned it for long-range rocket launches.

"While the test site appears to continue to be at a high state of readiness, it's anyone's guess when a detonation might occur. The North Koreans may do it tomorrow, some other day or they may decide to wait until a meaningful date like Kim Jong Il's birthday on Feb. 16," said Joel Wit, a former U.S. State Department official and the editor of 38 North.

Anniversaries related to members of North Korea's ruling dynasty, such as former leader Kim Jong Il, who died in December 2011, are marked with great fanfare in the authoritarian nation.

In its assessment of the preparations at Punggye-ri, 38 North noted that there was considerable snowfall there in November 2012. It said subsequent clearing operations as well as tracks in the snow in the remote area reveal activity at buildings and on roads near the possible test tunnel.

A satellite image taken Jan. 4 shows 30 or more personnel, possibly soldiers or security guards, in formation in the yard of the main administrative buildings at the site.

A Dec. 24 image shows a pile of material in the same yard. Its purpose is uncertain, but it could be intended for sealing the tunnel. By the time of Wednesday's photo, the pile had shrunk, which could indicate operations have begun to seal the tunnel, according to the analysis.