North Korea said it had successfully conducted a test of a miniaturised hydrogen nuclear device on Wednesday morning.
The announcement on North Korean state TV followed detection of a 5.1 magnitude earthquake near its known nuclear test site earlier on Wednesday.
The nuclear test is the fourth by the isolated country, which is under US and UN sanctions for its nuclear and missile programmes.
The US Geological Survey said it had measured an earthquake with a magnitude of 5.1 - bigger than the three previous bombs North Korea tested in 2013, 2009 and 2006.
A US military statement said the low-level flight over ally South Korea was in response to the North's 'recent provocative action'.
Tens of thousands of people danced and applauded in central Pyongyang, North Korea, in support of the country's reported H-bomb test.
Why is this latest development in North Korea's quest for nuclear power seen as potentially significant?