A Syrian rebel group disowned by al-Qaeda has demanded that Christians in a Syrian city it controls pay a gold levy, and curb displays of their faith in return for protection.

The Islamic State in Iraq and the Levant (ISIL) said that it would ensure Christians' safety in exchange for the levy and their adherence to restrictions on their faith, citing the Islamic legal precept of 'dhimma', Reuters news agency reported on Wednesday.

ISIL's directive to Christians in the eastern city of Raqqa is the latest evidence of the group's ambition to establish a state in Syria, a prospect that concerns Western and Arab backers of other rebel groups fighting Assad.

It said Christians must not make renovations to churches or other religious buildings, display religious insignia outside of churches, ring church bells or pray in public.

The statement published online demanded every Christian man pay a tax of up to 17g of gold, a levy that was common in Muslim states centuries ago.

The directive also bans Christians from owning weapons and from selling pork or wine to Muslims or drinking wine in public.

Mainly composed of foreign fighters, ISIL, which has been disowned by al-Qaeda, has been fighting President Bashar al-Assad, and is also engaged in a armed struggle with rival rebel groups.

The concept of dhimma, governing non-Muslims living under Islamic rule, dates back to the early Islamic era in the seventh century, but was largely abolished during the Ottoman reforms of the mid-19th century.

Strict interpretation of Islam

Raqqa was the first and only city to fall completely ISIL rebel control last year.

After repelling an offensive last month by rival armed groups and more moderate rebels, ISIL has turned its attention to setting up a state based on a strict interpretation of Islam.

It issued a more general set of restrictions for all residents of Raqqa last month, but this week's notice included the most extensive restrictions yet on Christians.

According to the Agence France Press news agency, of Raqqa's 300,000 population before the war, less than one percent were Christian. Many of the Christians later fled the city after ISIL started attacking and burning churches.

The statement, dated Saturday, was posted on a Twitter account of a purported ISIL supporter.

The text matched a statement distributed by the Syrian Observatory for Human Rights, an anti-Assad monitoring group, which condemned it.

The recognised al-Qaeda affiliate in Syria, Jabhat al-Nusra, has demanded ISIL submit to mediation to end the infighting that has killed about 3,300 people this year, saying it will "eradicate" them if they do not comply in the coming days.