Persecution watchdog Open Doors has called for a renewed focus on the plight of Christians in North Korea following the meeting between US president Donald Trump and North Korea's leader Kim Jong Un.

The country is number one on Open Doors' World Watch List of places where it is most difficult to be a Christian. Christians are severely persecuted and tens of thousands are held in harsh conditions in labour camps.

Zoe Smith, head of advocacy at Open Doors UK & Ireland, said: 'Open Doors estimates that around 70,000 Christians are interred in prison and labour camps, facing unimaginable torture, inhumane and degrading treatment purely because of their faith. The systematic persecution of Christians is just one of many heinous human rights violations perpetrated by the North Korean regime. If true change is to come to that country – and we hope it will – any further negotiations must confront the desperate human rights situation.'

The summit is being widely hailed as a success, though a joint statement issued afterwards contained few details about how progress toward denuclearisation might be achieved.

'It is unclear if further negotiations will lead to the end goal of denuclearisation,' Anthony Ruggiero, senior fellow of Washington's Foundation for Defence of Democracies think tank, told Reuters. 'This looks like a restatement of where we left negotiations more than 10 years ago and not a major step forward.'

The document also made no mention of the international sanctions that have crippled North Korea's economy for pursuing its nuclear weapons programme.

During his news conference afterwards, President Trump referred to the influence on the meeting of American student Otto Warmbier, who was released from North Korean custody in a vegetative condition and died shortly after his return to the US. 'Without Otto, this would not have happened,' Trump said.

He also said the subject of Christians in North Korea had been discussed and indicated that he was hopeful of change.

Speaking via Open Doors, John Choi (not his real name), a North Korean who escaped from the country and works as a human rights advocate in the UK, said: 'I hope that if there is economic advancement it will pave the way for more freedom for the people of North Korea, freedom of thought, opportunity and religion. I think this is likely to take 25 to 30 years but I also hope in light of Trump's comments today it will be much quicker than that.'

He added: 'This is the beginning of the process. The first steps have been taken. Trump hasn't clearly spoken about the human rights issues. But he has spoken about denuclearisation. Hopefully denuclearisation will lead to more money available to feed the everyday citizens of North Korea and provide them with a better life.

'President Trump said that the human rights issues are a continuing process. I am glad it is now on the agenda. But Kim Jong-un has to be committed to it too. Kim Jong-un has not yet referred to the prison camps or religious freedom. This is an ongoing process and I will continue to advocate and pray for it.'