An Algerian Christian convert has been sentenced to three years in prison for "insulting Islam" in a series of Facebook posts.

Slimane Bouhafs, 49, was sentenced on Tuesday by an appeals court in the Muslim majority country, after being arrested on July 31.

He was previously charged with blasphemy and sentenced to prison for five years on August 7; the maximum sentence for blasphemy.

Human rights campaigners are calling for his immediate release.

"Algerian courts have no business judging people's religious beliefs and opinions," said Sarah Leah Whitson, Middle East and North Africa director at Human Rights Watch (HRW).

"Algeria should urgently revise its penal code to stop criminalizing peaceful free expression, including views that may insult Islam and the prophet," she said.

The court's judgement said Bouhafs had "shared four distorted Koranic verses and photos offensive to the Prophet, as well as articles denigrating the Islamic religion," and that his conviction was on the basis of a series of online posts between May and June 2016.

These included "a caricature representing the Prophet Mohamed as a terrorist" and other posts "slandering Islam as a religion of intolerance and hatred."

According to HRW, Bouhafs shared on Facebook an open letter he had written to the secretary general of the UN, in which he denounced the "Islamisation of Algerian society" and the repression of Ahmadi Muslims and Christians.

HRW believes Bauhafs' rights were violated in court, and he was not informed of his right to a lawyer or the right to postpone his trial.

The Algerian League for the Defense of Human Rights has also condemned Bouhafs' verdict.

There are just 39,000 Christians among Algeria's population of more than 40 million, which is predominantly Muslim. The country ranks 37th on persecution charity Open Doors' list of places where it is most dangerous to be a Christian, and the charity has said the government is coming under increasing pressure to implement more Islamic legislation by Islamist groups.

Conversion, or an attempt to convert someone, from Islam is illegal and Muslim converts are forced to worship in secret. Only Muslims may hold public assembly and churches are often denied registration.