Seoul: Just hours after a 5.1 magnitude tremor was detected close to North Korea's nuclear test site, the state claimed that it had successfully conducted a hydrogen bomb test.
The announcement from the state came around 10:30 p.m. ET.
A state television reported "The republic's first hydrogen bomb test has been successfully performed at 10:00 am on January 6, 2016, based on the strategic determination of the Workers' Party."
A confirmed test would mark another big step toward Pyongyang's goal of building a warhead small enough to be mounted on a missile capable of reaching mainland America's shores.
North Korea conducted its third nuclear test in February 2013.
The hydrogen bomb test is likely to worsen Pyongyang's already bad ties with Washington and its neighbors.
It is expected to prompt a push for new, tougher sanctions at the United Nations.
Since the elevation of young leader Kim Jong Un in 2011, North Korea has ramped up angry rhetoric against the leaders of allies Washington and Seoul and the U.S.-South Korean annual military drills it considers invasion preparation.