North Korea announced it carried out its third test of a nuclear device underground in February. The test followed the December orbiting of a satellite using a long-range rocket, a trend coinciding with nuclear tests in 2006 and 2009.
John Hopkins University's North Korean forum 38 North said its review of satellite imagery of the Punggye-ri nuclear test site suggested possible new construction is under way. The site is where North Korea conducted tests in 2009 and 2013.
"This activity appears to have begun by late April 2013 and gathered momentum over the next few months," Johns Hopkins researchers said.
The forum stated that it is too early to determine what the new activity indicates, but it could be associated with construction or repairs tied to earlier tests.
"These activities do not appear to be part of preparations for a nuclear test in the near-term," the forum said. "Rather, they seem to be long-term projects ... that may be necessary for the conduct of future tests."