Emboldened militants, backed by Sunni tribal leaders, pushed toward Baghdad on Friday as Iran sent troops to fight alongside government forces. In Washington, increasingly nervous U.S. officials mulled their limited options to help slow the militants' advance.

In recent days, Iran has sent about 500 Revolutionary Guard troops to fight alongside Iraqi government security forces in Diyala province, a senior security official in Baghdad told CNN.

Meanwhile, Sunni tribal leaders have lined up behind radical Islamists from the Islamic State in Iraq and Syria, or ISIS, making their push toward Baghdad easier, a Saudi intelligence official told CNN's Nic Robertson.

Terrorists gain ground in Iraq fighting As Iraq further disintegrated, residents fled Mosul in droves. Militants captured the country's second-largest city this week after soldiers scattered, leaving their uniforms and weapons behind.

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