South Korea's military have played down the apparent successful test of a North Korean nuclear weapon, saying that that tremor it caused wasn't strong enough for a H-bomb.
A source, who asked to remain anonymous, told the Yonhap state news agency in South Korea that 5.1 magnitude earthquake apparently caused by the bomb was too slight to be a hydrogen device:
"It is hard to regard this test as that of a hydrogen bomb.
Only a few countries including the US and Russia have conducted hydrogen bomb tests and size of detonations reached 20 to 50 megatons."
The latest North Korean test amounted to around six kilotons and is "too weak" for a h-bomb, the official claimed.
But announcing the test, Pyongyang said it had successfully detonated a "miniaturised" device, which could explain its relatively small size in comparison to conventional nuclear weapons.
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?