North Korea could be preparing another missile launch, Seoul said Monday as it strengthened its defences following Pyongyang's biggest-ever nuclear test and declaration it had a hydrogen bomb.

The South and the United States will deploy more of the Terminal High-Altitude Area Defense (THAAD) missile launchers that have infuriated Beijing, the defence ministry said.

The announcement came after Seoul fired an early-morning volley of ballistic missiles in an exercise simulating an attack on the North's nuclear test site.

Pictures showed South Korean short-range Hyunmoo missiles roaring into the sky in the pale light of dawn from a launch site on the east coast.

Pyongyang said the device it detonated Sunday was a hydrogen bomb -- far more powerful than the fission-based devices it is believed to have previously tested -- and small enough to fit into a missile.

The blast threw down a new gauntlet to President Donald Trump, after the North in July twice tested an intercontinental ballistic missile (ICBM) that appeared to bring much of the US mainland into range, and threatened to send a salvo of missiles towards the US territory of Guam.

South Korean defence ministry officials estimated its strength at 50 kilotons -- five times the size of the North's previous nuclear test.

They did not confirm whether it was a hydrogen bomb, saying only that "a variety of nuclear material" had been used.

But Defence Minister Song Young-Moo said Seoul believed Pyongyang had succeeded in miniaturising its nuclear weapons to fit into an ICBM.

The South had requested the US deploy strategic assets such as aircraft carriers and bombers to the peninsula, he said, but denied reports Seoul was seeking the return of US tactical nuclear weapons.

Signs that North Korea was "preparing for another ballistic missile launch have consistently been detected since Sunday's test", the ministry said.

It did not indicate when a launch might take place, but said it could involve an ICBM being fired into the Pacific Ocean to raise pressure on Washington further.

- 'Appeasement' accusation -

After Sunday's test the United States warned it could launch a "massive military response" to threats from North Korea that would be "both effective and overwhelming".

"We are not looking to the total annihilation of a country, namely North Korea," Defense Secretary Jim Mattis said, but warned: "We have many options to do so."

Trump called an emergency meeting of his national security advisers and had his second telephone call of the weekend with Japanese Prime Minister Shinzo Abe.

But he did not talk to South Korea's Moon Jae-In for more than 24 hours -- instead accusing Seoul of "appeasement", raising jitters in Seoul about the two countries' decades-old alliance.

Moon, who advocates engagement as well as penalties to bring Pyongyang to the negotiating table, called for new United Nations sanctions to "completely isolate North Korea."

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."

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Kim Jong-Un: Absolute power -- and an H-bomb to wield it
Seoul (AFP) Sept 3, 2017
North Korean leader Kim Jong-Un ranks among the world's youngest heads of state, but has some of its most powerful weapons at his disposal. Sunday's test of what appeared to be a powerful, full-fledged thermonuclear bomb, marked yet another watershed in Kim's relentless drive as leader to turn the North into a credible - and feared - nuclear-armed state. In the process, he has simply s ... read more

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