(CNN)As the U.S. unveils details of a massive operation to reclaim Mosul from ISIS, a key Arab ally is warning the operation could further divide Iraq.
Key U.S. ally fears Iraq instability after anti-ISIS offensive
At issue is who will confront ISIS on the ground in a country where sectarian divisions run deep.
"Arab partners in the ISIS coalition are concerned that the planned operation by Iraqi and coalition forces to retake Mosul will be largely led by Shia-dominated Iraqi forces which will alienate local Sunnis and fuel instability after the operation", a senior Arab official tells CNN.
The country and its government are Shia-led, but Sunnis make up the majority in Mosul and other areas in northern and western Iraq where ISIS has been able to gain a foothold.
Asked if the administration shares these concerns, Deputy National Sec Advisor Ben Rhodes told CNN it does not, "because we work with multi-sectarian units so have the ability to know who we're coordinating with."
ISIS -- also referred to as the Islamic State or Daesh -- has long sought to capitalize on Sunni Muslims' disenchantment with the central government in Baghdad.
In recent months, pro-government Shiite militias have formed across the country to take on the terror group, but some have since been accused of committing atrocities against Sunni civilians.
In August, pro-government gunmen opened fire on worshipers at a mosque in Diyala province, killing 34 and wounding dozens. Some of them wore Iraqi security forces uniforms.
The U.S. could also face pressure from regional allies on another front. The senior Arab official who spoke with CNN says America's Arab coalition partners see "an urgency to confront the expanding ISIS threat in the region beyond Iraq and Syria."
The ISIS threat, this official says, "now stretches from Algeria to Afghanistan", far beyond the current battlefield.
Arab partners are particularly concerned about the terror group's growing presence in Libya, where just last week ISIS militants executed 21 Egyptian hostages -- an act that led Egypt to respond with a barrage of airstrikes in the beleaguered north African country.
And on Friday, ISIS claimed responsibility for a series of suicide attacks in the northwest city of Gobba.
While the U.S. has launched hundreds of airstrikes against the terror group in Iraq and Syria, but has not committed to any military action outside those two countries.
However, the military force authorization the administration is currently seeking from Congress would not limit future action geographically.
And at remarks in November at a Washington think tank, U.S. Secretary of State John Kerry said the U.S. "will remain committed to degrading and defeating Daesh in Iraq, in Syria, and wherever it is found. And we will do so until that goal is achieved, I assure you."