More than 100 Vietnamese Christian pastors have been locked up and are at risk of being poisoned for refusing to sign up to a central, official church, according to Christian Aid Mission.

It appears the aim of the official church was to control and diminish the influence of the thriving independent house churches. One group of house churches with a total membership of 3,000 was ordered to merge congregations and meet in a building that can hold just 500.

The jailed church leaders are from villages in Vietnam's central highlands. Many had fled the area or even the country but were forced to return and are now believed to be in jail.

Ministry leader Su, who himself served a term in prison between 1975 and 1984, said: "The government wants to combine them to limit their growth and have more control. If the pastors refused to sign a paper saying they would combine and that their gatherings would not go over 500 people, they would be beaten or thrown into prison."

Prison conditions are harsh and primitive, says Christian Aid Mission.

In rural areas, Sunday school is now banned along with youth groups and other gatherings. Children can only be taught Christianity at home, taking Communion and collecting offerings are forbidden, sermons are monitored by police to ensure they contain nothing that subverts communism and just one leader is allowed for each congregation.

The latest pastors to be imprisoned, more than 60, join another 42 who are still behind bars even though the 15-year sentences they received in 2001 for practising their faith have been completed. Their lands were seized when they were arrested.

Su said: "If you do not strictly follow the rules, they can put poison in your food. So you take a little bit, and if you feel something or want to vomit, you stop eating. A few people have died."

In spite of the risks, his team is continuing with their Christian mission among the villages of Vietnam. Missionaries are trained secretly, and Bible study groups move from village to village so they cannot be tracked down.

Gospel Herald reports that the 2016 Open Doors World Watch List of countries that persecute Christians, put Vietnam  at number 20 and gave it a maximum score for violence. Christians make up about one in 10 of the population.