FILE - In this April 15, 2012 file photo, a North Korean vehicle carrying a missile passes by during a mass military parade in Pyongyang's Kim Il Sung Square to celebrate the centenary of the birth of the late North Korean founder Kim Il Sung. The enormous, 16-wheel truck used to carry the missile, likely came from China in a possible violation of U.N. sanctions meant to rein in Pyongyang's missile program, experts say. Pinning a sanctions-busting charge on Beijing would be difficult, however, because it would be hard to prove that Beijing provided the technology for military purposes or even that it sold the vehicle directly to North Korea, the experts said. (AP Photo/David Guttenfelder, File)
New satellite imagery appears to show a train of mining carts and other preparations at North Korea's nuclear test site but no indication of when a detonation might take place.
Earlier this month, South Korean intelligence reported digging of a new tunnel at the Punggye-ri site, which it took as sign that North Korea was 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 282,500 cubic feet of rubble have been excavated at the site, where nuclear tests were conducted in 2006 and 2009.