Satellite imagery shows 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, a U.S. research institute said Wednesday.

That's the latest indication that Pyongyang is pressing on with its nuclear weapons program, although the U.S.-Korea Institute at Johns Hopkins School of Advanced International Studies says there's no sign another test explosion is coming soon.

In February, North Korea conducted its third and most powerful nuclear test since 2006, drawing international condemnation — including from its benefactor China — and tighter U.N. sanctions. The North now says it is willing to resume aid-for-disarmament negotiations without preconditions, but the U.S. remains skeptical of Pyongyang's intentions.

An analysis of recent commercial satellite imagery being published on the institute's website, 38 North, concludes there are two new tunnel entrances at the Punggye-ri site in the country's northeast, suggesting new tunnels are being constructed or new entrances for existing tunnels. There's also enlarged heaps of soil from excavations and construction probably intended to upgrade support buildings. The latest image is from Sept. 27.

"Concerns that North Korea could conduct a fourth nuclear test are justified given continuing excavation at the site. Pyongyang is probably making sure it is prepared although when another detonation might happen remains entirely unclear," said Joel Wit, a former State Department official and editor of 38 North.

It is notoriously difficult to divine the intentions of North Korea's secretive government. The images provide hints about what's going on at Punggye-ri but critical activities are conducted underground out of aerial view. The findings are based on analysis from Nick Hansen, a retired intelligence expert who closely monitors developments in the North's weapons programs.

Nuclear tests are viewed as critical for North Korea's efforts to miniaturize a nuclear device that can fit on a long-range missile capable of reaching America — a goal that most experts says could still be some years off.

North Korea is believed to have recently restarted a mothballed nuclear reactor that is capable of producing plutonium for bombs.