An Iranian high court has upheld the prison sentences of four Christians for converting to the faith from Islam.

The decision means Pastor Farhad Sabokrooh, of the Iranian Assemblies of God, his wife Shahnaz Jayzan, and church workers Naser Zaman-Dezfuli and Davoud Alijani have had to return to jail.

They were arrested in December 2011 after their church in the southern town of Ahwaz was raided by authorities during Christmas celebrations.

The four Christians were sentenced to one year in prison by the Revolutionary Court in Ahwaz for "converting to Christianity and propagating against the Islamic regime through evangelism".

They were temporarily released but were summoned to court on 1 May and re-arrested, Christian Solidarity Worldwide reports.

Mr Alijani was transferred to Ahwaz's Karoon Prison to complete his sentence, while Pastor Sabokrooh, Shahnaz Jayzan and Mr Zaman-Dezfuli were taken to Sepidar Prison.

CSW has warned of a crackdown on Christians since last December. Two other Assemblies of God pastors, Farshid Fathi and Saeed Abedini, are being held in Evin prison along with Church of Iran member Alireza Seyyedian and another Christian, Mostafa Bordbar, who was arrested in Tehran in December.

Pastor Abedini, a US citizen, was placed in solitary confinement for taking part in a peaceful protest against prison conditions but has since been released. CSW reports he also spent time in hospital due to his deteriorating health.

Mohabat News reports Vahid Hakkani, a Christian prisoner in Adel-Abad Prison in Shiraz, is in urgent need of medical attention. He was arrested in February last year along with eight others during a raid on a prayer meeting. He needs urgent surgery because of internal bleeding.

Ebrahim Firoozi was arrested for a second time in March and charged with launching and administering a Christian missionary website, distributing Bibles, cooperating with student activists and involvement in actions against national security. CSW reports he has been temporarily released from Evin Prison after a bail of around $20,000 was paid.

CSW's Chief Executive Mervyn Thomas said, "Pastor Sabokrooh and others have been jailed for exercising their right to change their religion and to manifest their new faith both privately and in communion with others.

"This right is protected by Article 18 of the International Covenant on Civil and Political Rights (ICCPR), which as a signatory, Iran is obliged uphold.

"We continue to call on Iran to fulfill its legal obligations by guaranteeing freedom of religion or belief for all of its citizens. We also urge the authorities to ensure that as occurred in the case of Pastor Abdedini, every prisoner in urgent need of medical attention is given access to medical facilities and appropriate treatment."