"It is still present in some places and its members are trying to infiltrate civilians and villages. IS has changed its method," he said.

"IS was defeated militarily in Iraq but not eliminated," said Abu Mahdi al-Mohandis, the number two in the Hashed al-Shaabi paramilitary force that has fought alongside the army against IS.

There are now fewer than 3,000 IS fighters left in Iraq and Syria, but they still pose a danger, according to the US-led coalition against IS.

Three years after proclaiming a cross-border "caliphate" stretching into Syria, the jihadists' brutal rule over a third of Iraqi territory is consigned to history.

Despite Iraq's declaration of victory over the Islamic State group, the jihadists still present a deadly threat and will hide deep in the desert or blend into the civilian population, experts say.

In a sign of the lingering threat, Iraqi forces said they killed 10 "IS suicide bombers" in a tunnel near the city of Kirkuk on Saturday.

After launching a sweeping offensive in 2014, the extremist organisation formed a proto-state as large as Italy straddling Iraq and Syria that was home to millions of people.

But now the jihadists have been forced back into hiding.

"Security is assured in the main cities because we are there, but it will not be complete without full control of the border wit

h Syria," Mohandis said.

On the Syrian side, the army and Kurdish fighters share control of the frontier.

In Syria, IS controls about eight percent of Deir Ezzor province and has a presence in Homs and Hama provinces as well as Yarmuk Palestinian camp near Damascus, according to the Syrian Observatory for Human Rights, a British-based war monitor.

'Not completely defeated'

In Iraq, the jihadists are expected to return to earlier tactics, according to the coalition.

"Daesh has been militarily degraded, but (they) are not completely defeated," said coalition spokesman Ryan Dillon, using an Arabic acronym for IS, also known as ISIS.

"They still pose a threat to Iraq," he added.

"As ISIS continues to lose land, influence, funding streams and conventional capabilities, we expect them to return to their terrorist roots by conducting high-profile attacks on helpless civilians such as those we've already seen in Nasiriyah, Ramadi, and elsewhere over the past weeks."

On September 15, gunmen and suicide bombers killed 84 people, mostly Shiite Muslim pilgrims, in Nasiriyah in southern Iraq.

The coalition also regularly reports raids in the Taji area north of Baghdad, the Qayyarah region south of Mosul and in the vast western province of Anbar.

The jihadists can also hide in the inhospitable desert south of the Euphrates River, said Hisham al-Hashemi, an Iraqi expert on IS.

"Fighting in the desert is very hard because there are valleys 12 metres (40 feet) deep and hundreds of kilometres (miles) long, and IS has dug hideouts impossible to detect by aircraft," he said.

"Every military unit that goes out there needs to be accompanied by an expert in cartography or it risks getting lost," he said.

The mopping-up operation has not started yet because a huge number of soldiers and Hashed fighters must be mobilised.

Among the areas to be combed are Wadi Hauran, the longest valley in Iraq stretching 350 kilometres from the Saudi border up to the

Euphrates River.

IS has established positions, hideouts, weapons depots and fallback positions in this rugged terrain, where there are gorges up to 200 metres deep.

"IS will return to its origins... in the desert, where there are caves that can serve as a refuge," said Iraqi security expert Fadel Abu Raghif.

"This is where it began."

Two years after its creation in 2006, the jihadist organisation then known as the Islamic State of Iraq was partly driven from its fiefdoms by the US military backed by Iraqi fighters.

It regrouped in the desert and went back on the offensive in 2014.

"Despite the success of the Iraqi forces, these (desert) regions cannot be totally secure," said Abu Raghif.