BAGHDAD — After enduring weeks of abuse by insurgents of the group called Islamic State, members of the Aza tribe struck a secret deal last month with local police and military officials: The authorities would supply weapons to two tribal regiments totaling about 1,150 fighters, and in return the tribe would help government security forces fight the Islamic State.

Several days later, the tribal regiments, in collaboration with Iraqi government troops and Shiite militia fighters, liberated 13 villages in Diyala Province from the Islamic State, which is also known as ISIS, officials said.

“ISIS has humiliated the top sheikhs of Diyala and has done horrible and unforgivable crimes against people here,” said Abu Othman al-Azawi, an Aza sheikh and a member of the provincial council. “They tried to vandalize the tribal system and break its ties.”

Despite the support of United States-led airstrikes, anti-Islamic State forces have had little success driving the insurgents from territory they seized in Iraq and Syria. On Friday, after weathering days of aerial bombardments, Islamic State fighters pushed into the Syrian Kurdish town of Kobani on the Syria-Turkey border.