North Korea's despotic leaders are determined to complete development of nuclear weapons by the end of 2017, according to the country's most senior defector.

Thae Yong-ho, 55, the former North Korean deputy ambassador in London, defected to South Korea earlier this year along with his wife and children due to his discontent with the regime and for their futures.

Mr Thae said Kim Jong-un will pursue his nuclear programme "at all costs" and will not relinquish its weapons "even if the country is offered $1 trillion or $10 trillion in return", Yonhap news agency reported.

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The election of Donald Trump and the impeachment of the South's president, Park Geun-hye, meant 2017 was an "opportune time" for the North, he said.

He added: "Due to domestic political procedures, North Korea calculates that South Korea and the US will not be able to take physical or military actions to deter North Korea's nuclear development."

Discussing his defection he said: "I made a vow to dismantle the Kim Jong-un regime and save our people from an approaching nuclear disaster.

"My heart aches when I think of my family members and colleagues who remain in North Korea. But regretting and crying will bring me nowhere. Fighting the Kim regime will facilitate unification of the Koreas."

James Clapper, the US director of national intelligence, has previously said trying to persuade North Korea to "denuclearise" is "probably a lost cause". Deputy Secretary of State Antony Blinken retorted by saying: "We will not accept North Korea as a nuclear state, we will not accept North Korea's possession of nuclear weapons. Period."

In November the United Nations Security Council imposed new sanctions on North Korea in response to its latest nuclear test two months prior.

It was the fifth and largest controlled detonation of a nuclear device ever undertaken by the pariah state.

The sanctions, which included a limit on coal shipments to China and a total ban on the export of statues, could reduce the North's overall export revenues by a quarter.

Mr Thae added in a brief interview with the BBC this week: "I am very happy now my family here is settling down, and everyone in my family thinks it is the right decision to come to South Korea for a new life. No regrets at all."

And he told Yonhap that while China, the North's main backer, could cripple the Kim regime, it does not act because of the geopolitical "buffer zone" it provides against US-allied Japan and South Korea.

His nuclear claim came as a report said Kim Jong-un has sent more than 300 people to their deaths since inheriting power in 2011, including 140 senior government officers.