Islamic State has set up departments to handle "war spoils", including slaves, and the exploitation of natural resources such as oil, creating the trappings of government that enable it to manage large swaths of Syria and Iraq and other areas.

The hierarchical bureaucracy, including petty rivalries between officials, and legal codes in the form of religious fatwas are detailed in a cache of documents seized by US Special Operations Forces in a May raid in Syria that killed top IS financial official Abu Sayyaf. Reuters has reviewed some of the documents.

US officials said the documents have helped deepen their understanding of a militant group whose skill in controlling the territory it has seized has surprised many. They provide insight into how a once small group has developed a complex bureaucracy to manage revenue streams - from pillaged oil to stolen antiquities - and oversee subjugated populations.