Pyongyang: The development marks a major advance in North Korea's nuclear weapons capabilities after Kim Jong-un said he is "ready for war".
North Korea claims to have successfully conducted a test of a its first hydrogen nuclear bomb .
The announcement was made on North Korean state TV after a 5.1 magnitude earthquake was detected near its known nuclear test site 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.
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The country added that it would continue to strengthen its nuclear programme in order to protect itself against the hostile policies of the United States.
In a statement, 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".
The test follows leader Kim Jong-un's claims last month that the country was in possession of its first H-bomb.
He said his late father and former leader Kim Jong-Il, "turned the DPRK into a powerful nuclear weapons state ready to detonate a self-reliant A-bomb and H-bomb to reliably defend its sovereignty and the dignity of the nation."
UK Defence Secretary Philip Hammond took to Twitter to condemn the actions of North Korea.
He wrote: "If North Korean H-bomb test reports are true, it is a grave breach of #UNSC resolutions & a provocation which I condemn without reservation."
The development marks a major advance in North Korea's nuclear weapons capabilities.
North Korea previously carried out tests on nuclear devices in 2006, 2009 and 2013.
