WASHINGTON (AP) — 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.