Tomas Ojea Quintana, the UN's special rapporteur on human rights in North Korea, called on Pyongyang to "issue a general amnesty to release hundreds of prisoners".

He hailed North Korea's recent release of three American prisoners, and urged the country to broaden its "amnesty" to anyone being arbitrarily detained there, which he said was basically all prisoners.

"There is no rule of law in the country... no due process of law," he said.

He said he understood prisoner releases would be part of a possibly drawn-out process and that Pyongyang would not throw open the doors to all of its prisons immediately.

But he said he thought "a good signal of the government would be to start releasing prisoners".

His comments came less than a week before unprecedented talks between North Korean leader Kim Jong Un and US President Donald Trump are scheduled to take place in Singapore.

The summit is due to be held following a rapid detente between Pyongyang and Washington -- as well as South Korea -- in a turn-around from a dramatic escalation of tensions last year, when the North ratcheted up its nuclear weapons programme.

Ojea Quintana insisted that human rights needed to be addressed as part of the security negotiations.

Earlier this year, Ojea Quintana said that any "denuclearisation deal will remain fragile if it sidelines the rights and needs of the DPRK population", using the official acronym for the country.

The isolated North has been accused of a litany of state-sanctioned rights abuses including extrajudicial killing, torture, brutal crackdowns on dissent and even kidnapping foreign citizens.

North Korea is estimated to have up to 120,000 political prisoners in its sprawling gulag system.

A UN commission published a searing report in 2014 which concluded North Korea was committing human rights violations "without parallel in the contemporary world".

In Ojea Quintana's latest report, he highlighted the use of torture in detention, chronic food insecurity and "severe restrictions on all forms of free expression, movement and access to information".