Islamic State (IS) has forbidden Christians from leaving its stronghold of Raqqa "under any circumstances", according to activists in the city.

According to the anti-IS group Raqqa is Being Slaughtered Silenced (RIBSS), the militant group issued an edict pronouncing the ban on Thursday.

"IS is preventing any Christians or Armenians who remain in Raqqa to leave," tweeted RIBSS.

There are reportedly only 43 Christian families left in Raqqa since IS imposed their strict brand of Islamic law on the city. Christians there live under a "state of subjugation" according to a document released by IS in December 2014.

"Fight those who do not believe in Allah, nor in the latter day, nor do they prohibit what Allah and His Apostle have prohibited, nor follow the religion of truth, out of those who have been given the Book, until they pay the tax in acknowledgement of superiority and they are in a state of subjection," said the document.

An activist with RIBSS said that the ban had been imposed at Easter, noting that most of the Christians left in the city were poor workers with little prospect of moving even if allowed the opportunity.

"They suffer from the decisions and practices of the organisation, which has occupied their places of worship while forcing them to pay tribute. They are also deprived from practicing their rituals and religious practices," said Abu Mohammed, adding they are also subject to regular harassment.

Christians at one point made up around 10 percent of Syria's population, but the rise of Islamic militant groups has forced hundreds of thousands to flee.

Those who remain are generally pro-government, since many see Syrian President Bashar al-Assad as the only bulwark against a rebel government they believe would institutionally discriminate against them.

IS have been accused of numerous atrocities against Christians in the areas they control in the Middle East. In the Iraqi province of Nineveh the group painted the Arabic letter for 'N' on the houses of Iraqi Christians before calling on them to convert, pay a tax or face death.

In a video released by the group in April 2015, IS was shown executing dozens of Ethiopian Christians in Libya.

In the video, an IS fighter delivers a statement about “Christian-Muslim” relations, superimposed by images of churches and altars being destroyed by IS.

The footage cuts to Christians in Syria, and explains how they were given the choice to either convert to Islam or pay a “special tax”. A text on the screen identifies the men as “followers of the cross from the enemy Ethiopian Church”.

“To the nation of the cross…Muslim blood that was shed on the hand of your religion is not cheap,” the fighter says in English. “We swear to Allah, you will not have safety even in your dreams until you embrace Islam."