U.S. intelligence officials have spotted activity at a key North Korean military facility that has them "kind of worried," a senior official told FOX News.
Support activity, including the movement of certain vehicles and personnel, has been spotted at Sanumdong, a research-and-development complex, the official said. The activity is consistent with that observed prior to the launch of the Taepodong-2 missile in the past.
The official said part of the concern, given the fact that Sanumdong is not itself a launch site, is that the North Koreans may be preparing to move a warhead by rail -- for an unknown purpose.
The WC-135 "Command Phoenix" Air Force plane that has been flying over the Korean peninsula has meanwhile collected a second air sample that could confirm the communist country's reported nuclear test Monday, the official said.
The sample is expected to arrive at McGuire Air Force Base by Thursday afternoon. The first sample was tested at Linde Corporation in Murray Hill, N.J., and will soon arrive at Los Alamos for further evaluation.
The sample will likely undergo another round of testing at the Pacific Northwest Lab in Seattle.
FOX News' James Rosen contributed to this report.