BAGHDAD—Militants killed 65 members of Iraq's Sunni minority in two attacks on Friday, one of them on a mosque, further inflaming sectarian tensions and imperiling an already fragile effort to form a unity government.

The attacks, which Sunni officials blamed on Shiite militia members, came as the governments in Baghdad and Washington are trying to forge a broad alliance against the militant forces of the Islamic State, the Sunni-led insurgency that has seized huge swaths of the country since June.

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