Barely a month after Baghdad declared victory over ISIS, the militants could still recapture areas of Iraq, especially near the border with Syria, experts and officials say.
Ali al-Bayati, a commander of the Popular Mobilization Forces which fought alongside Iraqi security forces in a grueling battle against the group, said the Nimrud area of northern Iraq could "fall at any time because security there is fragile".
Last July, the authorities in Baghdad announced the "liberation" of nearby Mosul, Iraq's second city and capital of Nineveh province.
ISIS militants who fled their former stronghold and took refuge to the west, in the vast desert towards the Syrian border, have since launched attacks on security forces and civilians, Bayati told Agence France Presse.
Hiding out in valleys and gullies as well as trenches dug before their ouster from Mosul, the militants have built up stockpiles of arms, fuel, water and food and pose a persistent threat to populated districts along the Tigris valley, like the Nimrud area downstream from Mosul.
More than 4,000 militants have been arrested in Nineveh province since Mosul's capture, according to police chief General Wathiq al-Hamdani.
But Aed al-Louayzi of Nineveh provincial council said several civilians have been robbed or killed inside the city itself, some by assailants disguised as soldiers.
He said the attacks have been the work of ISIS members from the areas of Tal Afar and Hatra, both towns also recaptured last year from the militants.
Louayzi said that "geographically, the territory has been retaken... but not all the militants there have been arrested".
"We are in the same security situation as that which led to the fall of Mosul" back in 2014, which came after the extremists had seized control of some areas, he told AFP.
Hisham al-Hashemi, a specialist on extremist movements, said Iraq's announcement in December of military victory "simply means that the ISIS flag is no longer flying" over government buildings.
To counter the threat of an ISIS resurgence, "several operations have been carried out south of Mosul" with US-led coalition support to seize arms, said coalition spokesman Colonel Ryan Dillon.
To try to avoid past mistakes, Dillon said, "the coalition has trained Iraqi security forces to address the transition and future threats. We knew there would be a transition from fighting to policing."
On Monday, twin suicide bombings in Baghdad cost more than 30 lives, prompting Prime Minister Haider al-Abadi to order security forces to "eliminate ISIS sleeper cells" and protect civilians.
But Hashemi said the threat is more immediate.
"This concept of sleeper cells is a mistake. They are not sleepers, they are active," he said. "They are capable of mounting attacks and even of taking control of zones."