According to the US Geological Survey, a 5.3 magnitude "explosion" has been detected in North Korea near the country's only known nuclear test site. North Korea has claimed to have tested nuclear weapons after similar seismic activity in recent years, but none as large as today's event.
South Korean defence officials have previously used seismic data to estimate the kiloton yield of North Korea's test weapons, but unknown variables like explosion depth make precise measurements impossible.
In January, North Korea announced it had obtained "the H-bomb of justice" after a 5.1 magnitude earthquake in the region. International experts, however, have contended that the tested device was likely a boosted fission weapon and not a true hydrogen bomb.
North Korea has not confirmed that this week's incident was a nuclear test, but, if so, it will be its fifth since 2006.