The three pastors and three church members were arrested and briefly detained on 6 October in the town of Wad Medani when they refused to hand over a school run by their evangelical church.
Five other churches – three belonging to the Sudan Church of Christ, one to the Presbyterian Church and the other to the Episcopal Church – have been told their buildings will be demolished.
Open Doors' source said officials told churches "their land had been assigned for investment".
According to the US Commission on International Religious Freedom, the Sudanese government "continues to engage in systematic, ongoing, and egregious violations of freedom of religion or belief."
Designated by the Commission as a 'country of particular concern' since 1999, Sudan's population is over 97 per cent Muslim, and the country's criminal code restricts religious freedom for all citizens. It also imposes Shariah Law on Muslims and Christians, allowing the death penalty for apostasy, stoning for adultery and prison sentences for blasphemy.
The USCIRF's 2015 report also notes the use of government policies and societal pressure to promote conversion to Islam. It is "impossible" to obtain permission to build churches, while their destruction has increased over the past four years.
According to Open Doors, security forces may legally stop any Christian worship or prayer meeting, and confiscate Bibles, documents and computers.