New guidelines released by the North Korea's Ministry of People's Security will result in increased surveillance and even more severe punishment for Christians throughout the closed country, according to one organisation.

Reverend Eric Foley, chief executive of US-based Seoul USA, which works to equip and prepare those who feel called to ministry in North Korea, says that four behaviours will be subject to heightened enforcement under the new guidelines.

These are: slandering leader Kim Jong-un, drug trafficking and consumption, distributing or viewing illegal recordings, and superstitious behaviour.

There are fears that these guidelines will be used against Christians, leading to increased fears for believers in the officially atheist nation which regards organised religion as a threat to the dictatorship.

"In North Korea, failing to give Kim Jong-un all glory and honour is the same as 'slandering' him," Foley explains.

"Underground Christians also use Christian videos bought in from outside the country for discipleship. And everything – from bowing one's head to possessing a Bible – is 'superstitious behaviour'."

North Korea is the worst country for the persecution of Christians according to Open Doors's annual World Watch List. It has retained its title for the 12^th year in a row, ranked as the single most anti-Christian country in the world.

The list is based on how free Christians are to practise their faith in the spheres of private, family, community, national and church life. In addition, there is a sixth sphere measuring the degree of violence present in the country. The spheres are all counted equally, with an aim to highlight structural persecution, rather than specific incidents.

The exact number of Christians in North Korea is unknown. Believers are forced to hide their faith under the Kim regime - worshipping and meeting in secret and even mouthing songs rather than singing out loud - but Foley estimates that there are around 100,000 underground Christians, a third of whom are incarcerated in concentration camps.

"That means two thirds have managed to avoid detection. Some of those would be believers who have been protected because of their high position or family history," Foley notes, although the recent execution of Kim Jong-un's uncle, Jang Song Taek, is indicative that position or blood no longer guarantee safety from the obsessive bid for supreme power by the dictator.

Kim is venerated by North Koreans, with the word 'Great' being added to his title soon after succeeding to the position of leader following the death of his father, Kim Jong-il, in 2010, and songs such as 'Can't live without him' are regularly played by the media, says Foley.

Despite this increase in persecution, and the constant threat of harsh punishment and even death, North Korean Christians remain steadfast in their faith, however.

Last year, Foley shared his surprise at the response he was given when he asked a member of the North Korean underground church how he could pray for them.

"You, pray for us? We pray for you ... because South Korean and American churches believe challenges in the Christian faith are solved by money, freedom, and politics. It's only when all you have is God do you realise God is all you need," the man said.

Christians in North Korea who have been able to speak with those outside the country have asked not to pray for them, "but instead to pray with them", Foley shares.

They ask "that God will empower us both to be faithful wherever he places us", he said.