A U.S. research institute says North Korea has made new tunnel entrances at its nuclear test site in a sign it is preparing to conduct more underground explosions there in the future.
But the U.S.-Korea Institute at Johns Hopkins School of Advanced International Studies says there's no sign that Pyongyang plans another test soon.
In February, North Korea conducted its third nuclear test since 2006, drawing tighter U.N. sanctions. It now says it is willing to resume aid-for-disarmament negotiations without preconditions.
The institute says recent satellite imagery shows two new tunnel entrances at the Punggye-ri site and enlarged heaps of soil, suggesting construction of new tunnels or new entrances for existing tunnels.
The findings were provided to The Associated Press Wednesday ahead of publication on the institute's website, 38 North.