(SEATTLEPI) — WASHINGTON (AP) — Commercial satellite imagery shows increased activity at North Korea’s nuclear test site but not enough to indicate an underground atomic explosion is imminent, a U.S. research institute said Tuesday.

North Korea last month threatened to conduct its fourth nuclear test and there’s been speculation it may do so as President Barack Obama travels to Asia this week.

The U.S.-Korea Institute at Johns Hopkins School of Advanced International Studies said that recent images, the latest taken Saturday, appear to show movement of crates and possibly lumber near tunnel entrances at the northeastern mountain site of Punggye-ri. But it said in an analysis published on its website — 38 North — that more movements of vehicles and equipment were detected in the weeks before previous detonations.

South Korea’s Defense Ministry also said Tuesday it has detected “various activities” at Punggye-ri, where North Korea has conducted three nuclear tests since 2006, the latest in February 2013.

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