North Korea said Wednesday it conducted a "successful" hydrogen bomb test.
The announcement on state television came after several monitoring agencies detected a man-made earthquake near a known nuclear testing site in North Korea.
The U.S. Geological Survey reported a 5.1 magnitude quake that South Korea said was about 50 kilometers from the Punggye-ri site where the North has conducted nuclear tests in the past. If confirmed, this would be the fourth such nuclear test by North Korea.
"We suspect a man-made earthquake and are analyzing the scale and epicenter of the quake with the geoscience and mineral resource institute of South Korea," a Korea Meteorological Administration official told the Reuters news agency by phone.
China's Earthquake Network Center described the unusual seismic activity as a "suspected explosion."
The South Korean news agency Yonhap said the foreign ministry in Seoul was holding an emergency meeting.
North Korea has conducted three nuclear tests: in 2006, 2009 and 2013 — all at the Punggye-ri site.
Researchers at the U.S.-Korea Institute at Johns Hopkins University said last month that recent satellite images showed North Korea was excavating a new tunnel at Punggye-ri.
"While there are no indications that a nuclear test is imminent, the new tunnel adds to North Korea's ability to conduct additional detonations over the coming years if it chooses to do so," they said at the time.
This story first appeared on Voice of America & is reposted here with permission.