North Korea has ramped up work at its nuclear test site over the last month, according to an analysis of satellite images released Tuesday, a day after the top U.S. envoy for North Korea warned Pyongyang that an atomic test would unify the world in seeking swift, tough punishment.

Glyn Davies’ comments after meetings Monday in Seoul with his South Korean and Japanese counterparts reflect widespread worry that North Korea may follow a failed April 13 long-range rocket test with its third nuclear test. Both of its previous nuclear tests, in 2006 and 2009, followed rocket launches.

Satellite images taken by DigitalGlobe and GeoEye in the past month show heightened activity at the Punggye-ri nuclear test site in North Korea’s northeast, including mining carts, excavation equipment and a large amount of debris taken from inside a tunnel and piled around its entrance, James Hardy, IHS Jane’s Asia Pacific specialist, said in a statement Tuesday. The most recent image was from May 9.

South Korean intelligence officials said last month that satellite images showed North Korea was digging a new tunnel in what appeared to be preparation for another nuclear test at the site. A new tunnel is likely needed because existing ones probably caved in and became contaminated with radioactive material after previous tests. (AP)