North Korean Christians are in danger if Donald Trump continues his aggression towards Kim Jong Un, a former missionary in the pariah state is warning.

Robert Park was tortured after crossing into North Korea on Christmas Day 2009. After two months of physical and sexual abuse he was released and has spent the intervening years campaigning against the regime.

He is warning North Korea's heavily persecuted Christian minority will be the first to suffer if Trump doesn't back down.

'Please kindly be reminded that a large number of underground Christians are within North Korea,' he wrote in an open letter to the President.

'They are the most persecuted religious group in the world, according to multiple watchdogs of religious rights internationally.

'As I pray your team accepts upon deep reflection, it would be decidedly un-Christian to countenance indiscriminate killings of those who are among the people in the world who suffer the most.'

He urged Trump to pursue a peaceful solution by supporting those within North Korea to oppose Kim Jong Un.

Park wrote: 'There is a thoroughly workable and peaceable solution to the North Korea crisis. It involves reaching out to the general populace of North Korea in sympathy and supporting their internal unseating of Kim Jong Un — one individual.

'This procedure must be accompanied by the freeing of all political prisoners — who are victims of crimes against humanity and possibly genocide — which can be achieved via the mediation of those North Koreans who assume interim administrative responsibilities in the immediate aftermath of Kim's indigenous and peaceful ouster.

'I've been praying through an outpouring of tears and wholeheartedly plead for you and your administration to remember the acute suffering and unparalleled victimisation of tens of millions of warm-hearted, gentle and benevolent North Koreans — who deserve compassion and require grace — and to please pursue a peaceable answer with regard to the security quandary.'