After seizing Christian communities in Syria, the extremist group Islamic State of Iraq and Syria (ISIS) is now forcibly suppressing Christianity in these areas.

ISIS militants have set commandments in a "safety contract" with Christians living in the captured Syrian town of Qaryatian, which they must follow to avoid being sentenced to death.

These commandments, directly issued by ISIS leader Abu Bakr al-Baghdadi, specifically prohibit practising the Christian faith, including the establishment of churches, the displaying of crosses, and saying Christian prayers out loud.

The ISIS also bans any offense against Islamic religious beliefs, including the sale of pork or wine to Muslims. Christian women were also required "to dress modestly."

Most notably, Christians were ordered to pay the "jizyah" or "jizya," or the tax imposed on non-Muslims. They were also warned against harbouring spies and carrying weapons.

Nahren Anweya, an Assyrian-American activist, said the ISIS issued these commandments to deter an estimated 250 Christians in Qaryatian from practicing their beliefs.

"Our entire ancestral homelands have been completely taken, and now they won't even allow us to sustain our religious faith in one God and his son Jesus Christ," Anweya said.

"We have been purged out of Mosul, Nineveh, Khabour, Hassaka, Qaryatian and many more ancestral Assyrian homelands. They took our native homelands, our girls, our churches and now they want the few lives we have left," the activist added.

Todd Daniels, Middle East regional manager for the group International Christian Concern, meanwhile said these commandments were ISIS' way of asserting supremacy over captured towns.

"The release of this agreement once again highlights the intents of the jihadists of ISIS to create a society in which only those who follow their strict interpretation of Islam have any sense of freedom. These restrictions nearly eliminate the presence of Christianity in this region," Daniel said.

He added that the ISIS is clearly violating the rights of the Christians to practise their religion.

"The campaign by ISIS extremists to establish their Islamic State and to drive out Christianity from Syria continues to amplify suffering and has caused the displacement of hundreds of thousands of Christians and more than 12 million Syrians. There will be no stable and lasting peace in Syria until citizens from all faiths are able to live together with all rights protected," Daniels said.