An Iranian court has turned down the appeal of four Christian converts who were sentenced to 10 years in prison last year for alleged involvement in house churches in the country.

According to Middle East Concern, the lawyer for the four defendants - Pastor Yousef Nadarkhani, Mohammadreza Omidi, Yasser Mossayebzadeh, and Saheb Fadaie - recently received a court order upholding the sentences handed down to his clients back in July 2017.

Apart from the 10-year prison sentence, Nadarkhani and Omidi reportedly received two-year internal exile. Both men were sent to the south of the country, far away from their families, with Nadarkhani being ordered to stay in Shahr, and Omidi being told to go to Borazjan.

The four men were initially arrested in May 2016 as they were taking communion. In September that same year, Mossayebzadeh, Omidi and Fadaie were sentenced to 80 lashes each for allegedly consuming communion wine. The three men have also filed an appeal on the case.

Nadarkhani, a member of the non-Trinitarian Church of Iran, was once sentenced to death for apostasy in 2009 after he questioned why only Islam was allowed to be taught in his children's school.

He was released on bail in July 2016, but he was accused of acting against national security later that year, according to World Watch Monitor.

The defendants appeared before the Revolutionary Court in December to appeal their conviction for propagating house churches and promoting "Zionist Christianity."

Niko Cordman from Middle East Concern had suggested at the time that the defendants were unfairly treated due to a law that Iranian-born Muslims are not allowed to convert to other religions.

"These men have not committed any crime, except basically living a Christian life and practicing the right to freedom of religion. That's the only reason they're being held accountable and taken to court," Cordman told Premier's News Hour at the time.

Miles Windsor of Middle East Concern has expressed concern that the jail terms being handed down against Christians are getting longer.

He said Christians in Iran are usually accused of "undermining national security," but recent cases suggested that the indictment was more closely connected to their faith.

"There was a hearing on the 11th of June last year. And a judge called Ahmadpour issued his verdict in July, sentencing Pastor Victor and Hadi Asgari to ten years imprisonment and Amin Naderi to 15 years imprisonment for conducting evangelism and illegal house church activity. So, these are charges which are entirely directed at their Christian faith," he told Mission Network News.

"Whilst Christians have consistently been put in prison for their faith in Iran in considerable numbers, the length of the sentence has seemed to have increased in the recent year or so," he added.