WASHINGTON (AP) ― New satellite imagery appears to show a train of mining carts and other preparations under way at North Korea’s nuclear test site but no indication of when a detonation might take place.

Early this month, South Korean intelligence reported digging of a new tunnel at the Punggye-ri site, which it took as a sign that North Korea was covertly preparing for a third nuclear test.

The U.S.-Korea Institute at Johns Hopkins School of Advanced International Studies provided the Associated Press on Friday with its analysis of a sequence of photos of the site obtained from a private satellite operator and taken between March 8 and April 18.

The analysis estimates that 8,000 cubic meters (282,500 cubic feet) of rubble have been excavated at the site, where the communist country conducted nuclear tests in 2006 and 2009.

“While it’s very clear from looking at these photos that the North has stepped up preparations for a nuclear test over the past few months, it’s unclear exactly when the blast will occur,’’ said Joel Wit, editor of the institute’s website, “38 North.’’