An elderly Christian man has been forced to convert to Islam in Raqqa, Syria, because he can no longer afford to pay the tax levied by ISIS on non-Muslims.

Islamic State's Amaq news agency released a video in which a man named Mostafa Abu Alzer recites the Shahada, the Muslim profession of faith. He then says that he has freely chosen to convert.

According to The Foreign Desk, Syrian sources said that Abu Alzer had previously chosen to stay in Raqqa, which has been a major stronghold for ISIS since it overran the city in January 2014, because he could afford to pay the 'jizya' tax, and wanted to stay to protect his home.

Christians in cities controlled by ISIS have been given the option to flee, convert to Islam, pay a tax or risk "death by the sword".

In 2014, ISIS released a list of seven rules dictating how Christians who still remained in Raqqa should behave.

The document said the jihadist group would guarantee the safety of Christians, and their "children, money and churches" provided the rules were followed.

Christians were forbidden from publicly showing their crosses, from building or repairing churches and from openly reciting the Bible or performing acts of worship.

They were also warned not to "make a mockery" of Islam, and not to stop Christians from converting.

According to activist group Raqqa is Being Slaughtered Silently, (RBSS) there are now fewer than 50 Christian families living in the city.

In March, ISIS forbade all Christians and Armenians from leaving Raqqa under any circumstances.

A spokesperson for RBSS cited concerns that they may be used as "human shields or hostages".