North Korea's Punggye-ri nuclear test site appears "primed and ready" for the country's sixth atomic test, an analysis by a U.S. research organization said amid growing concerns over Pyongyang's weapons programs.

The influential 38 North website, a project of the U.S.-Korea Institute at Johns Hopkins University, said Wednesday that commercial satellite imagery of the site taken the same day showed "continued activity around the north portal, new activity in the main administrative area, and a few personnel around the site's command center."

But South Korean military officials said there have been "no unusual" signs detected in connection with reports of nuclear or missile tests, the South's Yonhap news agency reported Thursday.