Four Christians, who were convicted of "promoting Zionist Christianity" in Iran, are preparing to start serving their 10-year prison sentences after their appeal was rejected earlier this month.

According to the London-based advocacy group Article 18, a pastor and three members of the Church of Iran are expected to report to the prison in the city of Rasht as soon as they receive a call from the government.

World Watch Monitor reported that the four Christians - Pastor Youcef Nadarkhani, Yasser Mossayebzadeh, Saheb Fadaie and Mohammad Reza Omidi - appealed their sentenced on Dec. 14 but it was rejected by the Revolutionary Court.

Mossayebzadeh, Fadaie and Omidi were reportedly arrested on May 13, 2016 for taking communion wine. They were sentenced to 80 lashes each in September that same year for wine consumption. Nadarkhani and his wife, Tina, were also arrested on May 13, but were released that same day, according to a previous report from World Watch Monitor.

Nadarkhani and Omidi were also sentenced to two years' internal exile, in addition to their 10-year prison terms.

Four other Christians are also awaiting a decision on their appeals at the Revolutionary Court over their lengthy prison sentences.

Article 18 asserted that the judge who heard the appeals of both groups of Christians "doesn't have a good track record in dealing with Christians arrested for their Christian activities."

"Iranian Christians are concerned about the unjust verdicts issued against Pastor Victor Bet-Tamraz, and three other Christians who were sentenced alongside him, Amin Afshar Naderi, Kaviyan Fallah Mohammadi and Hadi Asgari," Mansour Borji of Article 18 told World Watch Monitor.

"We are following the appeal process closely and ask all Christians worldwide, and the key members of the international community, to join us in calling for these convictions to be overturned. The Iranian government has to be reminded of its obligations under international law and its own constitution, to end its harassment of the peaceful Christian community," he continued.

A court handed down a 10-year prison sentence to Bet-Tamraz and three converts in July last year for "acting against national security by organising and conducting house-churches." Naderi reportedly received an additional five-year prison sentence for blasphemy.

Last year, Iranian courts handed down lengthy prison sentences ranging from 10 to 15 years to more than a dozen Christians, with most of the convicts being converts to Christianity.

Watchdog group Open Doors has ranked Iran in the 2018 World Watch List as the 10th nation where Christians experience the most persecution.

The U.S. State Department has also included the country in the list of Countries of Particular Concern (CPC) beginning in 1999 "for having engaged in, or tolerated, systematic, ongoing, and egregious violations of religious freedom."