Intelligence and local officials said that, a few months ago, they noticed a growing stream of commanders and fighters flowing out of the city to the Hamrin mountains in northeast Iraq which offer hideouts and access to four Iraqi provinces.
Some were intercepted but many evaded security forces and began setting up bases for their new operations.
What comes next may be a more complex and daunting challenge for Iraqi security forces once they finish celebrating a hard-won victory in Mosul, the militants' biggest stronghold.
Intelligence and security officials are bracing for the kind of devastating insurgency al Qaeda waged following the 2003 U.S.-led invasion, pushing Iraq into a sectarian civil war which peaked in 2006-2007.
"They are digging in. They have easy access to the capital," Lahur Talabany, a top Kurdish counter-terrorism official, told Reuters. As part of the U.S.-led coalition, he is at the forefront of efforts to eliminate ISIS.
"I believe we have tougher days coming.”
Some Iraqi ISIS fighters have roots dating back to al Qaeda's campaign of car and suicide bombs that exploded by the dozens each day and succeeded in fueling a sectarian bloodbath in Iraq, a major oil producer and key U.S. ally.
When a U.S.-funded tribal initiative crushed al-Qaeda, the hardcore regrouped in the desert between Iraq and Syria. They reappeared with a new jihadist brand that took the world by surprise: ISIS.
Shortly after its lighting sweep through Mosul, the group outdid al Qaeda's brutality, carrying out mass beheadings and executions as it imposed its ultra-hardline ideology.
Unlike al-Qaeda, it seized a third of Iraqi territory, gaining knowledge of land that could come in handy as it hits back at Iraqi security forces.
source: reuters