Photo shows work at N-test site

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 US-Korea Institute at Johns Hopkins School of Advanced International Studies provided an 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 metres (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 38 North website.

North Korea has already drawn United Nations Security Council condemnation for a failed, long-range rocket launch on April 13 which tried to put a satellite into orbit but was viewed by the US 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 lies in the country's north east, 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 long-time 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 was armed with "powerful modern weapons" capable of defeating the US - a claim questioned by experts.