Over a dozen Christians have been re-arrested in house raids in Eritrea.
All of the 15 Christians arrested have been previously imprisoned for their faith, some for up to 16 years, says Release International.
The men and women range in age from their late 20s to 60s, and come from different churches.
Most have already served sentences of between five and six years, and were released from prison last summer.
According to Release, they have been re-arrested following the discovery of a list of Christian contacts and are now being held in the Mai Serwa maximum-security prison in the Eritrean capital, Asmara.
It follows the re-arrest in July of pastors Girmay Araia and Samuel Gebrewleldi. No reason was given for the arrest of the pair, who are both in their 70s and lead the Full Gospel Church of Eritrea. They are still being held in Asmara's Second Police Station.
It brings the total number of Christians being detained in Eritrea to around 160.
Release CEO Paul Robinson is calling for their release.
"Release International continues to press the government to release its prisoners of faith – every one of them," he said.
"We urge Eritrea to repeal its restrictive religious laws and grant full freedom of faith to its citizens."
Christians have suffered brutal persecution in Eritrea since a crackdown was launched against churches in 2002, when most of them were closed.
Since then, many believers have been imprisoned in harsh conditions.
Some have been held in shipping containers in the desert that are freezing during the night and unbearably hot during the day.
There had been some optimism for the country's Christians after a number were released from prison over the past year, but the recent arrests have quashed these hopes.
As a result of the persecution, many Christians have fled the country to neighbouring Ethiopia where they are now at risk once again due to the conflict between Tigrayan forces and soldiers from Ethiopia and Eritrea.
Release is supporting Christian refugees from Eritrea.
"There was good news last year when some Christians were set free," said Release partner, Dr Berhane Asmelash.
"We live in hope, but the government has yet to change its policy. We have been here before."