Sudanese authorities have kept six Christian leaders under surveillance and two detained since mid-December, despite having no official charges filed against them, according to World Watch Monitor (WWM).

Telahoon Nogosi Kassa Rata, a leader of Khartoum North Evangelical church, and Rev Hassan Abduraheem Kodi Taour of the Sudan Church of Christ (SCC), have been detained beyond the legal limit under Sudanese law without charge.

Five other pastors who have previously been arrested and released must report each day to Sudan's National Intelligence and Security Service (NISS).

Rata was initially detained "on religious charges", however sources close to the case have reportedly suggested the Christian activist is being investigated for espionage.

Sudanese law limits detention to 45 days without charge. Rata was arrested on December 14 and Taour on December 19.

Rata has not been seen since 2015. He has been kept incommunicado and is denied visitation rights.

Taour has been held in an undisclosed location since his arrest.

He was taken from his home in Omdurman, west of Khartoum, on the same day that the head of a SCC church committee, Rev Kuwa Shamal, was taken from his home in the district of Bahri.

Shamal was released days later, however he is required to report daily to the NISS and forced to stay at NISS offices for extended periods of time.

"The government of Sudan continues to intimidate and harass Christians and tries to make it difficult for them to practice their faith, and stop their ministries in their communities," Kamal Fahmi, head of advocacy website 'Set My People Free', told WWM.

Fahmi said these latest arrests and surveillance measures are part of a larger campaign against the Christian community in Sudan.

"Since the secession of South Sudan [in July 2011], Khartoum has intensified the war in Blue Nile and the Nuba Mountains, along with the indiscriminate harassment and arrests of church leaders and active church members," he said.

"Foreign Christian workers have been deported. Sudan has stopped the import of Christian literature and scriptures, while confiscating most of the Christian literature in the country and closing the only Christian bookshop in the capital Khartoum.

"Torture and arrest of converts from Islam is also commonplace," he added.