Some 27 Christians have been granted conditional release from prison on Eritrea.
Christian Solidarity Worldwide (CSW) said the release was understood to be contingent on the submission of property deeds making their guarantors liable for their future actions.
They were released from Mai Serwa Prison, near the capital city of Asmara on 4 and 8 September, possibly as part of measures to contain Covid-19.
Their release follows the arrest two weeks earlier of several Christians in Asmara, including four church leaders.
One source told CSW that the imprisonment and release of Christians was a "government strategy" to instil fear.
"They cannot detain everybody, so they keep you for some time, hoping that you will become weak or frightened," they said.
"Then they put in other people. They release and put other people in prison at the same time."
Tens of thousands of people are being held withour charge or trial across Eritrea, many of them in terrible conditions.
CSW's source estimates the number of incarcerated Christians to stand at a little over 300, including 39 children.
Some prisoners are kept in shipping containers, others in underground cells or in the open air in the desert. The prisons are overcrowded and unsanitary, and there is little medical care for the prisoners.
CSW's Founder President Mervyn Thomas called for the release of all prisoners of conscience.
"While applauding the fact that people who were deprived of their liberty have regained their freedom, it is also important to recall that they were detained arbitrarily and without due process for excessive periods simply on account of their religious beliefs," he said.
"Moreover, these releases remain conditional, as they were secured by property deeds, leaving the guarantors vulnerable to losing their properties.
"The guarantors could also lose their freedom should a former detainee exercise the right to leave the country, a right articulated in Article 12 of the International Covenant on Civil and Political Rights, to which Eritrea is party.
"Far more prisoners of conscience remain arbitrarily detained than have been released, and the fact that these releases were preceded by further arrests is indicative of an ongoing repression of the right to freedom of religion or belief.
"CSW therefore continues to call for the immediate and unconditional release of prisoners detained arbitrarily, particularly in view of a pandemic that poses a risk to life for those still held in inhumane conditions."