The US has flown a B-52 bomber over South Korea in a show of force after North Korea said it successfully tested a hydrogen bomb.
The B-52 flew low over an air base close to the North-South border.
Pyongyang said it detonated an underground device on Wednesday, to widespread condemnation, but experts remain sceptical about the claim.
North Korean leader Kim Jong-un said the test was an act of self-defence to prevent nuclear war with the US.
"It is the legitimate right of a sovereign state and a fair action that nobody can criticise," he was quoted by North Korean state media as saying.
Following the test, South Korea resumed broadcasting propaganda from speakers mounted on the border and the North countered with similar broadcasts.
The two sides are technically still at war, with the civil conflict that ended in 1953 concluding in a truce rather than a peace treaty.
The US is an ally of the South and said the B-52's flight was a response "to recent provocative action by North Korea", reportedly.