A Sudanese pastor who was arrested in December 2015 and subsequently released has been re-arrested, according to Christian Solidarity Worldwide (CSW).

Rev Kowa Shamal, a minister of the Sudan Church of Christ, was originally arrested by Sudan's National Intelligence and Security Service (NISS) in December 2015 alongside his colleague Rev Hassan Abduraheem and Christian activist Talahon Nigosi Kassa Ratta.

Shamal was released in January 2016. In February conditions were added to his release, forcing him to report to NISS on a regular basis. He was also made to hand over his passport, laptop and other personal items, according to CSW.

On May 24, he reported to NISS as usual, but was arrested and his case is now with the prosecutor.

"We are deeply concerned by the re-arrest of Reverend Kwa Shamal. The Reverend has complied for months with NISS's requirement that he reports daily to their offices, even though he had not been charged with any crime," said CSW chief executive Mervyn Thomas.

This follows the transfer of Abduraheem from the detention of NISS to the Attorney General's custody on May 9.

Lawyers have confirmed that a criminal investigation has been initiated against Abduraheem, who may be charged with national security crimes, and that Shamal has been added to the investigation.

The lawyers expressed concern at the conditions the two were being kept in, reportedly in small rooms with intermittent electricity and limited ventilation.

"His treatment and that of Reverend Abduraheem are indicative of a government campaign to harass and restrict the rights of the Christian community," said Thomas.

"These developments and the ongoing detention of Reverends Abduraheem and Shamal also raise concerns regarding Sudan's compliance with fair trial principles, articulated in the 2005 interim constitution and the country's international obligations."

Both pastors were arrested at their homes. Although family members were given no reason, the pastors were among those who have objected to churches being bulldozed. Shamal's church in Hai Thiba Al Hamyida was demolished in the summer of 2014. Ratta, a member of Sudan Evangelical Presbyterian Church, has campaigned against the confiscation of church buildings.

"We urge the authorities to end the judicial harassment of the Christian community and to uphold the right to freedom of religion or belief for every citizen. We also call on the Sudanese authorities to rectify any derogation from constitutional and international fair trial principles and ensure that the men are detained in a safe and habitable environment," said Thomas.