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.