Iraqi and American security officials say that territory in the north could easily be recaptured by ISIL despite Baghdad’s insistence that the extremists have been defeated.
In an attempt to repel a comeback, major military operations are being conducted near Mosul where towns risk falling back into insurgent hands.
Although victory was declared against ISIL in December, the group remains the world’s most active terror organisation by number of attacks.
An independent report on Thursday said such incidents are escalating even though terror deaths have declined.
The instability comes as the country's politicians squabble ahead of elections scheduled in May.
Illustrating the threat from ISIL, the northern city of Nimrud could "fall at any time because security is so fragile", according to paramilitary leaders.
Ali Al Bayati, commander of Hashed Al Shaabi, who fought alongside Iraqi troops in routing the extremists, said ISIL could easily recapture areas near the border with Syria.
“Fighters who have fled their former stronghold of Mosul and taken refuge to the west, in deserts toward the Syrian border, have launched attacks on security forces and civilians,” he said.
Officials say civilians have been killed and others robbed inside the city, some by assailants disguised as soldiers -- a long used hallmark of insurgent attacks since a US-led coalition toppled dictator Saddam Hussein in 2003.
“Geographically, the territory has been retaken... but not all the jihadists there have been arrested,” said Aed Al Louayzi, a lawmaker in Nineveh province.
Iraq faces a similar security situation which led to the swift fall of Mosul to ISIL in 2014, he said.
Jane's Terrorism and Insurgency Centre, based in London, said Thursday that Iraq bears the brunt of insurgents' wrath.
ISIL killed 6,499 people in attacks last year , a 40 per cent fall compared with 2016, despite assaults rising nine per cent to 4,612, the report said.
Attacks have recently rocked the northern towns of Tal Afar and Hatra, both recaptured last year from the extremist group.
To eliminate a resurgence “several operations have been carried out south of Mosul" with US-led coalition support to seize arms, said US Army Colonel and coalition spokesman Ryan Dillon.
The Iraqi commander of military operations in Nineveh, General Najim Al Jubouri, said that "troops alongside the US-led coalition against ISIL are coordinating attacks against extremists."
Fighters have built up stockpiles of arms, fuel and food while hiding out in valleys and trenches in northern areas.
Michael Rubin, resident scholar at the American Enterprise Institute, in Washington, said ISIL could exploit the country's political problems.
"They will seek to exploit local grievances and weak militaries. If prime minister Haider Al Abadi fails to provide economic opportunities in liberated areas, Iraq could face an increasing challenge," he told The National.
In Hawija, a northern city in the disputed oil-rich Kirkuk province, at least three civilians and a Hashed fighter have been killed this month, according to security sources.
They said around 60 extremists have died in fighting around the town, one of the last ISIL urban strongholds retaken by Iraqi forces.
Violence continues to hit Baghdad, where twin suicide bombings killed 38 people on Monday in what was declared the “deadliest attack” since Mr Al Abadi said ISIL had been beaten.
The bombings in the capital led him to order security forces to "eliminate ISIL sleeper cells" and protect civilians.
But the prime minister is embroiled in a difficult political balancing act.
Chaos emerged in parliament on Thursday as lawmakers failed to confirm May 12 as the election date. A law to ensure the vote also is yet to be passed.
Many MPs boycotted the chamber as they want the election postponed to allow millions of displaced people to return home and cast their ballots.
The American embassy in Baghdad appealed to Iraq's politicians to resolve their disputes.
"Postponing the elections would set a dangerous precedent, undermining the constitution and damaging Iraq’s long-term democratic development," the embassy said, criticising calls to delay the vote.
Mr Al Abadi is seeking a new term as the head of a “Victory Alliance” while Hashed Al Shaabi’s political bloc will stand separately. Both aim to gain seats in parliament, citing their role in combating ISIL.
The premier, who has vowed to rise above sectarian divisions by leading a mixed list of candidates in the polls, launched a new appeal Thursday for potential allies to join his alliance.
May's polls are the fourth parliamentary and provincial assembly elections since Saddam's ouster.