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.
In Seoul, ministry spokesman Kim Min-seok told reporters that the South Korea military was keeping in mind that North Korea can conduct a nuclear test suddenly, or could be just pretending to prepare for one "and deceive us, like they did in the past."
In Washington, a U.S. defense official said Tuesday that there have been signs of increased activity but had no details. The official was not authorized to discuss the matter by name and spoke on condition of anonymity.
State Department spokeswoman Jen Psaki said the U.S. was closely monitoring the situation on the Korean Peninsula and continued to urge North Korea "to refrain from actions that threaten regional peace and security."