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.

For example, one diwan, roughly equivalent to a government ministry, handles natural resources, including the exploitation of antiquities from ancient empires. Another processes "war spoils", including slaves.

"IS is invested in the statehood and Caliphate image more so than any other jihadist enterprise. So a formal organization, besides being practical when you control so much contiguous territory, also reinforces the statehood image," said Aymenn al-Tamimi, an expert on structure of IS.

The documents also show how "meticulous and data-oriented" IS is in managing the oil and gas sector, although it is not a sophisticated operation, said Amos Hochstein, the US State Department's top official for energy affairs.

US officials said the documents have helped the anti-IS coalition to pinpoint vulnerabilities.

The documents show the Islamic State is not immune to the rivalries and personality clashes that typify bureaucracies everywhere. A Nov 21, 2014 letter from the Diwan of Natural Resources emphasizes that Abu Sayyaf is in charge of handling antiquities.

"The reason being is that he is very knowledgeable in this field and that Abu Jihad al-Tunisi is a simpleton who can't manage the division," it said.