The UN Security Council met to discuss more sanctions on North Korea after it claimed to have denoted its first hydrogen bomb.
It said it violated UN resolutions and was destabilising to the region.
Doubt has been cast over whether it was a hydrogen bomb, but if true it could mark a major upgrade in the country's capabilities.
ITV News International Affairs Editor Rageh Omaar reports:
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?