North Korea has apparently carried out its fifth - and largest ever - nuclear test, South Korean officials say.
The report comes after a 5.3 magnitude earthquake was detected by North Korea's main nuclear site.
South Korean President Park Geun-hye described the test as "self-destruction" and "maniacal recklessness" on the part of North Korea.
In the wake of the test, North Korea state TV declared: "We've improved our ability to produce standardized and miniaturized nuclear warheads so we can produce as many as we want.
"This is our response to hostile powers, including the US. We sent out a message that if the enemies attack us, we can counterattack.
"The measures to protect our dignity and our right to exist from the US threat will continue."