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.