N. Korea could be preparing new missile launch: Seoul

But Trump criticised the US treaty ally on Twitter, saying: “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!”

The nuclear test prompted an international chorus of condemnation. The Security Council will hold an emergency meeting on Monday.

At their summit in China, the North’s key ally, the five-nation BRICS grouping — taking in the host nation as well as Brazil, Russia, India, and South Africa — said Monday it “strongly deplores” the test.

Moon and Abe agreed to work for stronger sanctions against the North, but seven sets of UN measures have so far done nothing to deter Pyongyang.

US Treasury Secretary Steven Mnuchin said Sunday his department was preparing measures to “cut off North Korea economically” and ensure anyone trading with it could not do business with the US.

That would impact Beijing, which is responsible for about 90 percent of the North’s commerce, but would also have dramatic consequences for the US as China is the world’s second-largest economy.

– ‘Perfect success’ –

On Sunday US monitors measured a powerful 6.3-magnitude earthquake near the North’s main testing site, felt in parts of China and Russia, with an aftershock possibly caused by a rock cave-in.

According to the South’s Yonhap news agency, Seoul’s National Intelligence Service said it was the fifth blast the North had conducted in the same No 2 tunnel at the Punggye-ri test site, and it was “likely to have collapsed”.

But it said the North had already completed construction of a third tunnel, so that it could carry out another test at any time it chose, and work was underway on a fourth.

The North hailed the test as “a perfect success”.

Despite fears of a possible radioactive leak after the apparent collapse, Japanese and Chinese scientists said they had detected no radiation in the atmosphere.

Hours before the test, the North released images of leader Kim Jong-Un inspecting a device it called a “thermonuclear weapon with super explosive power” entirely made “by our own efforts and technology”.

The respected 38 North website urged caution, saying it was likely the item pictured was “only a model mock-up”.

The North says it needs nuclear weapons to defend itself against the threat of invasion, and analysts say it is seeking to strengthen its hand for any future negotiations with Washington.

An increasing number of experts say talks are now the only way to address the issue.

“The Americans need to open a direct channel at a high level with the North Koreans,” John Delury of Yonsei University in Seoul told AFP. “That’s the missing piece to all of this.”