The images of the site where nuclear tests were conducted in 2006 and 2009 show 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.
However, it remains difficult to determine North Korea's true intentions since a test would be conducted underground.
The Associated Press received this analysis from 38 North, the website of the U.S.-Korea Institute at Johns Hopkins School of Advanced International Studies. The most recent image was taken on Wednesday.
Thursday, North Korea's powerful National Defense Commission declared its plans for a nuclear test, describing it as part of "upcoming" action but not explaining exactly when or where it would take place.
According to 38 North, the nuclear test site, which is located 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 said some trucks have been seen moving around the site. One official said the U.S. is not ruling out the possibility that a test could happen soon. 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.
The Associated Press contributed to this report.