Three Iranian Christians are due to appear before Iran’s Islamic Revolutionary Court on February 4 to appeal against their 2017 convictions on charges of "conducting evangelism" and carrying out "illegal church activities."

Pastor Victor Bet Tamraz, Amin Afshar Naderi, and Hadi Asgari each received a provisional sentence of 10 years in prison for the charges filed against them.

The Islamic judges also imposed an additional five-year prison sentence against Naderi on a charge of blasphemy.

The Revolutionary Court has the power to end the case, confirm the sentences, or refer the case to Iran's Supreme Court.

Iran's secretive Revolutionary Court system was established after the 1979 Islamic Revolution to try suspected ideological opponents of the country's Islamic rulers.

It operates without juries, without defense lawyers, and often without evidence except confessions extracted through torture.

The court system's case load includes suspects charged with espionage, threatening the internal and external security of Iran, trying to overthrow Iran's Islamic government, blasphemy, inciting violence, and drug smuggling.

On February 2, four UN human rights experts issued a joint statement calling on Iran to ensure "a fair and transparent final hearing" for the three men.

The UN special rapporteurs said the charges and sentences were "completely contrary" to Iran's obligations under international law.

"We are additionally concerned about the lack of health care made available to them while in detention and, in particular, about the current health condition of Mr. Asgari, who remains in prison," they said.

The UN experts also said they were concerned that the prosecution of the three Christians was not an isolated case.

They called on Iran to "ensure fair trials for all, including the religious minorities in the country."

"We are aware of several other reported cases in which members of the Christian minority have received heavy sentences after being charged with 'threatening national security,' either for converting people or for attending house churches," their joint statement said.

"This shows a disturbing pattern of individuals being targeted because of their religion or beliefs, in this case a religious minority in the country," they said.

"Members of the Christian minority in Iran, particularly those who have converted to the faith, are facing severe discrimination and religious persecution," the UN experts said.

The UN experts also urged Iran's government to "immediately and unconditionally release all those who have been arrested and detained for exercising their right to freedom of religion or belief."

The United States said in its 2017 International Religious Freedom Report that Tehran "regulated Christian religious practices closely to enforce the prohibition on proselytizing."

With reporting by AFP and BBC