Possible two-stage hydrogen bomb seen 'game changer' for North Korea

North Korean leader Kim Jong Un provides guidance on a nuclear weapons program in this undated photo released by North Korea's Korean Central News Agency (KCNA) in Pyongyang September 3, 2017. KCNA via REUTERS

SEOUL (Reuters) - North Korea's claim of a successful hydrogen bomb test marks a major step in the isolated country's long-stated goal of developing a nuclear-tipped missile that puts the U.S. mainland within range, experts say.

North Korea conducted its sixth and most powerful nuclear test on Sunday, which it said was a successful detonation of an advanced hydrogen bomb, technically known as a two-stage thermonuclear device.