Sanliurfa, Turkey: The Syrian businessman was enjoying a much-needed holiday in Turkey when the phone call came from the tax inspector of the Islamic State.

His business partner in Raqqa had been arrested, the inspector told him, and he would not be released until he paid the $131,000 ($US100,000) that it owed the "caliphate".

"They told me that because I have a lot of money, I have to pay my share," said "Ammar", who asked for his real name not to be used. "They analyse your income and take a percentage."

As IS, also known as ISIL or ISIS, establishes its empire, the jihadists have become fastidious bureaucrats - imposing taxes, paying fixed salaries and introducing trading standards laws to try to create a healthy economy that will sustain their autocratic rule.

Yet despite brutal punishments for those who break the rules, many Syrian businessmen prefer it to the anarchy that prevails in areas controlled by other rebel groups, including the Western-backed Free Syrian Army.