South Korea has announced it will temporarily deploy additional launchers for a US THAAD missile defence system as its Defence Ministry said there were signs North Korea was preparing for more ballistic missile launches.

Key points: South Korea will not say when additional launchers would be moved into place

South Korea will not say when additional launchers would be moved into place Defence Ministry says there are signs North is preparing for "more ballistic missile launches"

Defence Ministry says there are signs North is preparing for "more ballistic missile launches" China lodges "solemn representations" with Pyongyang over claimed hydrogen bomb test

The deployment decision was made by President Moon Jae-in in the face of growing tension sparked by North Korea, which conducted its sixth and most powerful nuclear test on Sunday.

Some construction of the controversial Terminal High Altitude Area Defence (THAAD) system will be carried out to deploy the four launchers at the site in Seongju, south of Seoul, the Defence Ministry said in a statement.

There are currently two launchers at the location, a former golf course. The ministry did not specify when the launchers would be moved onto the site.

Pyongyang claimed the latest test was an advanced hydrogen bomb with the capability of being attached to an intercontinental ballistic missile.

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The claim could not be independently verified, but the US Geological Survey did record a magnitude-6.3 tremor near where the North claimed to have conducted the test.

The rollout of the THAAD system, vehemently opposed by neighbouring China, had been delayed since June.

South Korea's announcement of a boost to its defences came as it warned the North appeared to be preparing for further provocations.

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"We have continued to see signs of possibly more ballistic missile launches," Defence Ministry official Chang Kyung-soo said.

"We also forecast North Korea could fire an intercontinental ballistic missile."

South Korea's air force and army conducted exercises involving long-range air-to-surface and ballistic missiles on Monday, the joint chiefs of staff said, adding more drills were being prepared with US forces in the South.

Asian leaders push for dialogue as US talks tough

Sorry, this video has expired Mattis threatens 'massive military response' if North Korea attacks

North Korea's main ally China said it had lodged "solemn representations" with the embassy in Beijing over the latest missile test.

China's Foreign Ministry said Pyongyang was clear about Beijing's opposition to North Korea's nuclear test and reiterated that talks were needed to resolve the issues on the Korean Peninsula.

South Korean President Moon Jae-in and Japanese Prime Minister Shinzo Abe agreed to pursue stronger UN sanctions in light of the North's provocations.

"Both heads of state agreed to cooperate closely with each other and the United States and shared the understanding there must be the most powerful sanctions and pressure applied on North Korea," presidential Blue House spokesman Park Su-hyun said.

The aim of stronger sanctions was to draw North Korea into dialogue, he said.

Meanwhile, neutral Switzerland has said it is prepared to act as a mediator to help resolve the crisis, including holding ministerial talks, adding that sanctions had not been effective to date.

"I think it really is time for dialogue," Swiss President Doria Leuthard said, but added that China and the United States had to take their share of responsibility while warning of "over-reactions".

"We are ready to offer our role for good services as a mediator … because, well, Twitter won't be an adequate instrument … this must be very discreet."

In a series of tweets on Sunday, US President Donald Trump appeared to rebuke the idea of aiming for dialogue, saying that "talk of appeasement with North Korea will not work, they only understand one thing!"

Meanwhile, the Pentagon renewed its warning of a "massive" military response if it or its allies were threatened by the North.

"We are not looking to the total annihilation of a country, namely North Korea," US Defence Secretary Jim Mattis said after meeting Mr Trump and his national security team.

"But as I said, we have many options to do so."

China's National Nuclear Safety Administration said data from radiation monitoring stations near the North Korean border showed no impact on "China's environment or populace".

Sorry, this video has expired Mr Turnbull called on China to impose pressure on North Korea.

AP/Reuters