His comments came hours after the US Geological Survey picked up a 6.3 magnitude "explosion" in North Korea, which Pyongyang confirmed was a nuclear test, its sixth.

The isolated regime said this one was of a hydrogen bomb that could be fitted atop a ballistic missile, sharply raising the stakes in a US-North Korea confrontation.

Trump last month threatened North Korea with "fire and fury" if it continued to threaten the United States, but he refrained from direct threats in his latest tweets.

"South Korea is finding, as I have told them, that their talk of appeasement with North Korea will not work, they only understand one thing!" he said.

"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." (AFP)