At a time of warnings about possible impending war between the two nations, researchers from the group at John Hopkins University’s US-Korea Institute said fresh images appear to show that Pyongyang could be preparing for a fresh missile test.
The images come from the Punggye-ri site and were taken late last month.
They appear to show workers pumping out water at a tunnel believed to have been prepared for an upcoming nuclear test.
It also apparently shows some soldiers possibly playing volleyball.
The researchers said they were trying to interpret what exactly was going on at the site.
“It is unclear if this activity indicates that a nuclear test has been cancelled, the facility is in stand-by mode or that a test is imminent,” said the team at John Hopkins University.
They said the latest images were “unusual” and could even be part of a North Korean misinformation effort.
North Korea has carried out an estimated five nuclear tests over the past decade or so.
The secretive country said earlier this week that it was prepared to carry out a nuclear test “at any time and at any location”.
It comes at a time of increased tension between Washington and North Korea.
US President Donald Trump recently warned of a “major, major conflict” between the two countries.
He also said that he would be prepared to meet North Korea’s leader Kim Jong-Un in person to try and resolve any future conflict.
It was also reported on Wednesday that China, North Korea’s biggest ally, had told its citizens to leave the country.