North Korea's nuclear weapons development may be designed to take over arch-rival South Korea and coerce the United States into abandoning its close ally, a senior White House official has said.

Questioning the North's stated purpose of warding off a US invasion, Matt Pottinger, the Asia director on President Donald Trump's National Security Council, said there may be some truth to claims that the North wants a nuclear deterrent to protect its communist dictatorship.

But he said the country's robust conventional military has worked as a deterrent for decades.

Mr Pottinger suggested other "disturbing" explanations for the North's development of "an arsenal of the worst weapons in the world".

He said: "They have made no secret in conversations they have had with former American officials, for example, and others that they want to use these weapons as an instrument of blackmail to achieve other goals, even including perhaps coercive reunification of the Korean Peninsula one day."

The North, he added, also wants to coerce the United States "to leave the peninsula and abandon our alliances".

His comments at a panel discussion organised by Sasakawa USA, a group promoting US-Japan relations, came a day after Mr Trump opened the door to a future meeting with North Korea's Kim Jong Un, offering unusual praise for the globally ostracised leader at a time of surging nuclear tensions.

The US has sent warships to the region to deter North Korea from conducting another nuclear test to advance its weapons programmes.

The North's nuclear and ballistic missile development already threatens South Korea and Japan, and within years could put the US mainland within striking range. The North also has a formidable array of conventional artillery and rockets trained on the heavily-populated South Korean capital.

Echoing other Trump administration officials, Mr Pottinger said the US is not seeking regime change in North Korea. Rather, he said the US wants an end to North Korea's nuclear weapons programme.

"We really have no choice but to increase pressure on North Korea to diplomatically isolate them, to bring a greater economic pain to bear until they are willing to make concrete steps to start reducing that threat," he said.

Meanwhile, officials in South Korea have said a controversial US anti-missile system in the country is now operating and can defend against North Korean missiles.

The Terminal High Altitude Defence (Thaad) system set up at a converted golf course in Seongju, in the country's south-east, has ''early capability'' to respond to North Korea's nuclear and missile threat, Seoul's Defence Ministry spokesman Moon Sang Gyun said.

He did not say when Washington and Seoul expects Thaad to be operating fully.

But the deployment has triggered anger from Seongju residents who fear North Korea may target their town and worry about rumoured health hazards linked to Thaad's powerful radar.

AP

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