There are reports that imprisoned Pastor Saeed Abedini has been placed in solitary confinement in an Iranian prison.
Christians around the world have been praying for the release of the pastor, who has dual US-Iranian nationality.
Pastor Abedini, a convert to Christianity, was arrested last September while visiting family in Iran and was sentenced to eight years in prison on 27 January for "actions against the national security of Iran" and "planting house churches that are intended to undermine national security".
He was sent to Evin Prison, where he has reportedly suffered beatings and been denied medical treatment by prison authorities.
Mohabat News reports that Pastor Abedini and nine other prisoners were placed in solitary confinement after taking part in a peaceful protest over the mistreatment of inmates in the prison.
Pastor Abedini also signed a petition criticising the lack of medical care for prisoners and the mistreatment of their families during visits. Prison authorities are reportedly no longer allowing family members to visit the pastor.
Chief Executive of Christian Solidarity Worldwide, Mervyn Thomas, said he was deeply concerned that Pastor Abedini is being held in solitary confinement despite urgently needing medical attention.
He raised concerns about another imprisoned pastor, Behnam Irani, imprisoned in similar conditions without access to medical treatment.
"We urge the Iranian authorities to provide immediate medical care for Pastor Abedini and Pastor Irani, and to release them and all other prisoners who in reality are being held on account of their faith," he said.
"The freedom to profess one's faith is protected in the Iranian constitution as well as in various international treaties to which Iran is signatory, including the International Covenant on Civil and Political Rights (ICCPR).
"We therefore call on Iran to fulfil its legal obligations and uphold this right for all of its citizens."