SEOUL, South Korea — 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.