Pyongyang said it had successfully tested a hydrogen bomb and praised the test as a “perfect success.” It says the bomb can be loaded onto an intercontinental ballistic missile of the kind it recently fired over Japan.

South Korea’s weather agency says the detonation set off a magnitude 5.7 earthquake, indicating a nuclear blast yield of 50-60 kilotons, making it at least five times stronger than the country’s last nuclear test in 2016.

The announcement, coming as Chinese President Xi Jinping opened the Brics summit in the southern city of Xiamen with a blistering attack on US economic protectionism, caused immediate furore.

Beijing expressed “strong condemnation” and warned Pyongyang to “stop taking actions which worsen the situation.”

It urged the North to “consider the steadfast will of the international community” which was unanimous in opposing its nuclear tests and said it would continue to implement UN security council resolutions — which have slapped punishing sanctions on North Korea — in order to advance the “complete denuclearisation of the Korean peninsula.”

China has been seeking a de-escalation of tensions on the peninsula which would see North Korea stop testing bombs and firing missiles in return for an end to US and South Korean military manoeuvres in the area, that the North says are dry runs for an invasion.

After repeated condemnations of the North’s behaviour, South Korea’s President Moon Jae In was clearly running out of superlatives, saying vaguely he would meet the challenge with the “strongest measures possible” and press the US to deploy “the strongest strategic assets” it has to “completely isolate” Pyongyang.

His office would not clarify what this meant but it is unlikely to renew Western interest in the Chinese proposal, which Washington rejects because it insists the responsibility to reduce tensions rests exclusively with North Korea.

Moscow added its “strongest condemnation” to a chorus of anger around the world at the development.