Revd Shamal had been reporting daily to the National Intelligence and Security Service (NISS) offices as a condition of his release, following his initial arrest in December 2015 during a crackdown by the authorities on Christians in the country.
He was re-arrested by the police on 24 May and his case is now with the prosecutor. CSW are concerned he may be charged with "national security crimes".
Christian Solidarity Worldwide's Chief Executive Mervyn Thomas said: "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.
Thomas said the news was indicative of a government campaign to "harass and restrict the rights of the Christian community".
He added: "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."
Revd Shamal's arrest follows the transfer of his colleague Revd Hassan Abduraheem from the detention of NISS to the Attorney General's custody on 9 May.
Lawyers representing Revd Abduraheem have expressed concern at the conditions in which the men are being detained, noting that the clergymen are being held in small rooms with intermittent electricity supply and limited ventilation.
Revds Shamal and Abduraheem are from the Nuba Mountains in South Kordofan, an area where since 2011 the Khartoum government has conducted an indiscriminate campaign of aerial bombardment in its war against the Sudan People's Liberation Movement, causing multiple civilian deaths and destroying humanitarian structures.
