SEOUL (Reuters) - North Korea conducted a nuclear test on Tuesday, South Korea's defence ministry said, after seismic activity measuring 4.9 magnitude was registered by the U.S. Geological Survey. The epicentre of the seismic activity, which was only one km below the Earth's surface, was close to the North's known nuclear test site. "We've been informed by the South Koreans that there's been a (North Korean) nuclear test," a U.N. Security Council diplomat told Reuters on condition of anonymity. ...