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.
North Korea said on Wednesday it would continue to strengthen its nuclear programme in order to protect itself against the hostile policies of the United States.
In a statement after conducting its fourth nuclear test, North Korea's state news agency said it will not give up its nuclear programme as long as the United States maintained what it called "its stance of aggression."
North Korea also said it will act as a responsible nuclear state and vowed not to use its nuclear weapons unless its sovereignty was infringed. It also said it will not transfer its nuclear capabilities to other parties.