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."
North Korea already has drawn U.N. Security Council condemnation for a failed, long-range rocket launch April 13 which tried to put a satellite into orbit but was viewed by the U.S. and other nations as a cover for a test of its ballistic missile technology. Pyongyang could face tougher sanctions if it goes ahead with a nuclear test.
Punggye-ri site lies in the country's northeast, and the analysis says the images show various activities at the site since March. The latest photo shows a train of mining carts, which are believed to be used to carry material excavated from within the test site.
The size of the spoil pile appears unchanged in the latest image, and it is unclear whether the test device has been placed in the chamber and the shaft sealed with other material for the final preparation stage before a detonation, the analysis says.
North Korea's longtime ruler Kim Jong Il died in December and was succeeded by his youngest son, Kim Jong Un. The North has stepped up its tough rhetoric against rival South Korea and the United States since the failed rocket test that blemished its commemorations of the centennial of the birth of the nation's founder, Kim Il Sung.
On Wednesday, a top military chief in Pyongyang said the North is armed with "powerful modern weapons" capable of defeating the U.S. — a claim questioned by experts.
Washington worries about the possibility that North Korea might develop a reliable intercontinental ballistic missile and wed it with a nuclear bomb. Outside experts say the North has enough plutonium for about four to eight "simple" bombs, but does not yet appear to have the ability to make bombs small enough to mount on a missile.