Irbil, Iraq: Leaders of Iraq's Sunni Muslim tribes threatened on Friday to rebel against the Islamic State, the first indication that a change of government in Baghdad might allow a new prime minister to rally the country's divided ethnic and religious groups against the Islamist extremists.

But the Sunni offer to battle the militants came with strings – possible autonomy and the withdrawal of Iraqi military forces from Sunni areas – that would be difficult for a Shiite-led government to grant, and Shiite politicians in Baghdad showed little enthusiasm.

Refugees: Iraqi Yazidis in the Newroz camp, Syria, after fleeing from the Islamic state. Credit:AFP

US officials have predicted since the Islamic State began its sweep through much of central, western and northern Iraq, often with the collaboration of Sunni tribes, that a more conciliatory government in Baghdad, coupled with harsh rule imposed by the Islamists, would move disaffected Sunnis to rebel.

That is one reason the US pushed so hard for prime minister Nouri al-Maliki to resign in favour of a replacement who would be more disposed to offer concessions to Sunnis and Kurds.