US President Donald Trump says North Korea's "words and actions continue to be very hostile and dangerous" to the United States.
Trump confirmed Twitter that "North Korea has conducted a major nuclear test" on Sunday.
"North Korea is a rogue nation which has become a great threat and embarrassment to China, which is trying to help but with little success," Trump said in a second tweet.
The nuclear test prompted US National Security Adviser HR McMaster to hold 20-minute emergency phone talks with his South Korean counterpart, Chung Eui Yong, according to the Yonhap news agency.
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South Korea's military said it had raised its alert level and put its nuclear crisis team into operation.
The magnitude of North Korea's test was estimated at 5.8 and the explosive yield was approximately eight times more powerful than the 1945 Hiroshima bomb, a Norway-based geological monitoring agency said.