North Korea “has ignored the international community’s widespread opposition, again carrying out a nuclear test. China’s government expresses resolute opposition and strong condemnation toward this”, the foreign ministry said in a statement on its website.
The test on Sunday was widely felt in northeast China and rocked some cities for as long as eight seconds, according to reports and accounts on social media.
The tremor was felt as far away as the city of Changchun around 400 km (250 miles) northwest of the North’s test site at Punggye-ri, according to state broadcaster CCTV.
In the small city of Yanji, some 20 km from the border, some people reported the shaking was so intense that they fled their homes.
Mr Jiemiao Cangxin, a commentator on the Chinese microblog Weibo, said his building swayed so much that “I put my underpants on and I just ran, and when I reached the first floor, I can say I wasn’t the only one running away with just my underpants on!”
“In Yanji, we felt the shaking for ten seconds,” said Weibo user Buziranshaonv.
“I was lying down and sleeping when the tremor woke me up. At first, I thought it was a dream,” said another.
The test, North Korea’s sixth, was substantially larger than previous ones, measuring 6.3 on the Richter scale, according to US monitors.
That would make it between five to six times larger than Pyongyang’s previous effort in September last year, according to South Korea’s weather agency.
The North called it a test of a hydrogen bomb which was a “perfect success”.
Reports of the explosion also provoked widespread concern further away in China, with many commentators speculating about the timing of the event – just hours before Chinese President Xi Jinping is due to open a summit of BRICs nations in southern China.
“An earthquake happened in North Korea, everyone thinks at once it is a nuclear test,” said one user, cheekily adding “Is it a form of greetings for the summit in Xiamen?”
President Xi, in his speech at the Xiamen summit, made no mention of North Korea.