A prominent human rights group is calling for the release of two South Sudanese pastors who are currently detained in Sudan.

The two South Sudanese pastors, Yat Michael and Peter Yen, also known as David Yein Reith in other reports, of the South Sudan Presbyterian Evangelical Church, have been in custody of the National Security and Intelligence Services (NISS) of Sudan for more than a month now.

They are allegedly being held in a secret location, and have been unable to communicate with their families.

Amnesty International issued a statement last week calling for the release of the two pastors, since they have not been properly charged for any offence.

"To date, absolutely no charge has been brought against these two church leaders," the group's UK Director Kate Allen said. 

Allen then called on Sudanese authorities to reveal the pastors' whereabouts and offences.

"The authorities in Sudan must reveal the location of these two men as soon as possible," Allen demanded. "[A]nd either charge them with a recognisable criminal offence or release them immediately."

The NISS arrested Pastor Michael in December 21. According to Morning Star News, the NISS took Michael into custody after a prayer service at the Khartoum Bahri Evangelical Church. Although originally of the South Sudan Presbyterian Evangelical Church, Pastor Michael was invited to the Khartoum Bahri Evangelical Church to provide encouragement as the church was facing demolition at the request of Muslim investors seeking to take over its property.

Pastor Peter Yen was arrested by NISS officials on January 9. A separate report by the Morning Star News stated that NISS called Yen while conducting a prayer service. The article said that NISS officials demanded that Yen return home immediately or his family will be arrested.

The arrests were possible since the NISS is authorised to make arrests of non-Muslims without the legal requirement of a formal charge.

In her statement, Allen stressed the need for urgent action to release the two pastors.

"The longer these two men are held in a secret location, the greater the risk of them being tortured," she said.