Iraqi Prime Minister Nuri al-Maliki has vowed to crush illegal armed groups in Baghdad "regardless of sect or politics."
But the so-called Islamic State in Iraq said the plan was an Iranian ploy against Sunni Muslims.
The group's leader, Sheikh Abu Umar al-Baghdadi, urged followers to be ready to "repulse the attacks."
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SUNNI, SHI'A: Iraq is riven along sectarian lines, faults that frequently produce violent clashes and are a constant source of tension. Sectarian concerns drive much of Iraqi politics and are the main threat to the country's fragile security environment.