Two South Sudanese pastors facing criminal charges in Sudan have been moved to a high security prison and have not been allowed any visitors, their lawyers have confirmed.

Rev Yat Michael and Rev Peter Reith were previously detained at the low security Omdurman Men's Prison, and were last seen by their relatives and pastors from their church on June 3. When their families returned on June 4, they were refused access, and authorities confirmed two days later that the men were now at Kober Prison in North Khartoum.

According to Christian Solidarity Worldwide (CSW), no one has been allowed to visit either Michael or Reith and there is speculation that the lockdown is on the orders of the National Intelligence and Security Service (NISS).

Michael was arrested on 14 December 2014, and Reith in January of this year. They were both detained without charges, and without access to a lawyer or their families, until March 1. They are now being held on six charges including espionage, "offending Islamic beliefs" and undermining the constitutional system. Two of the charges carry the death penalty or life imprisonment.

The men maintain they have not committed any crime. Michael recently told CBN news from his prison cell that he didn't know why he had been arrested: "We just go to to out ministry training in our church".

Pastor Reith (who has been named as David Yein in some reports) said that despite his treatment, "I am not afraid of anything".

"I am never afraid of anything because it is my love... because I believe. God chose me to suffer," he said.

The men's next hearing is due to take place on June 15.

Chief executive of CSW, Mervyn Thomas, said he is concerned by the latest development in the clergymen's case.

"They already endured extended detention without access to their families at the beginning of this year and they and their families should be spared further emotional distress," he said in a statement.

"Moreover, the refusal of access to their legal representatives is in violation of article 14 of the International Covenant on Civil and Political Rights (ICCPR), to which Sudan is party, and which guarantees the right of those charged with a crime to communicate with counsel of their own choosing.

"We urge the Sudanese authorities to ensure that the clergymen's detention is regularised, and they are permitted regular family visits and unhindered access to their lawyers. The decision to detain them at a higher security prison should be reconsidered, given that they have not been found guilty of any crime."

Freedom of religion or belief is guaranteed in Sudan's constitution, but it is ranked sixth on Open Doors' World Watch List of countries where Christians face the most persecution. It has been designated a 'country of particular concern' by the US State Department since 1999.