BAGHDAD: A week after his tank division punched through Islamic State defenses on the southeast edge of Mosul, an Iraqi army colonel says the fight to drive the militants out of their urban stronghold is turning into a nightmare.

Against a well-drilled, mobile and brutally effective enemy, exploiting the cover of built-up neighborhoods and the city's civilian population, his tanks were useless, he said, and his men untrained for the urban warfare they face.

His Ninth Armoured Division and elite counter terrorism units fighting nearby seized six of some 60 neighborhoods last week, the first gains inside Mosul since the Oct. 17 start of a campaign to crush Islamic State in its Iraqi fortress.

Even that small foothold is proving hard to maintain, however, with waves of counter attacks by jihadist units including snipers and suicide bombers who use a network of tunnels stretching for miles (km) under the city.

They appear able to strike at will, often at night, denying the troops rest and rattling frayed nerves.

"We're an armored brigade, and fighting without being able to use tanks and with soldiers unused to urban warfare is putting troops in a tough situation," the officer told Reuters. He asked not to be named because he was not authorized to talk to the media.

A year ago, when his forces took part in an operation to drive Islamic State from the much smaller city of Ramadi west of Baghdad, they were tasked with holding territory outside while the counter terrorism forces entered the city.

Mosul, whose capture is a crucial step towards dismantling the caliphate Islamic State declared two years ago across large areas of Iraq and Syria, is too big for specialist forces alone.

"In Mosul, we have to advance inside residential areas, comb streets, clear houses from terrorists and deal with civilians. I'm afraid this job is too tough for us to handle".

He said it was impossible to differentiate between civilians and fighters who melt in amongst them. Islamic State has forced its dress code on the population during the two years it has controlled the city. Men are required to have long beards, something the militants are still policing.

"Our soldiers can't recognize them until it's too late, when the attacker either detonates his explosive vest or throws a grenade," the colonel said, adding that he lost two T-72 tanks and an armored vehicle in a single day's fighting on Tuesday.

"It's becoming a nightmare and it's nerve-wracking for the soldiers," he said.

TOUGHEST URBAN WAR

Even for the Counter Terrorism Service, or special forces, trained more specifically for the challenges in Mosul, the last week of fighting has been unprecedented.

"We are carrying out the toughest urban warfare that any force in the world could undertake", CTS spokesman Sabah al-Numani said on Sunday.

One CTS officer, in Baghdad on leave, told Reuters the biggest threat came from snipers. "You don't know where or when a sniper will strike," he said. That, combined with thousands of people trying to escape the fighting, was a constant source of stress.

As he spoke, a voice on his radio crackled - one of his men on the frontline. "Sir, there are so many civilians, they have these suitcases with them as well. How do I know what's in them? And they're coming towards me..."

Islamic State leader Abu Bakr al-Baghdadi, who declared a crossborder caliphate in Syria and Iraq from the pulpit of a Mosul mosque two years ago, told his fighters last week there could be no retreat in a "total war" with their enemies.

Hisham al-Hashemi, who advises the Iraqi government on Islamic State issues and has visited the frontlines, said all the indications from Mosul so far showed that Baghdadi's comments were no idle threat.

"Now Daesh (Islamic State) is really fighting," he said.

Hashemi said the jihadists had dug a 70 km (45 mile) network of tunnels just on the eastern side of the Tigris River, which runs through the center of Mosul, since they took over in 2014.

Using the tunnels they were able to surprise troops inside the city, striking between 2 am and dawn when their defenses are at their lowest. "They are not ready for these surprises - it's the tunnels which have caused our greatest losses," he said.

"CRASHING WAVES"

Hashemi said government forces were only in full control of two of the districts they entered last week.

The army says it has captured five other districts, but fighting continues in all of them and Hashemi said in some neighborhoods the army had been driven back three or four times - often at night - before reclaiming territory the next day.

With its tanks unable to navigate narrow city streets, the Iraqi army has called on US Apache helicopters to target car bombers. The Pentagon said on Monday they would continue to be used "in what we expect will be tough fighting to come".

One of the most devastating tactics the militants employed, which helped them tie down a far greater force than their own, was to send consecutive waves of small units - about 50 strong - against the troops so they could never let down their guard.

The militants call the operation "crashing waves". Each unit includes suicide bombers, snipers, assault fighters, and what they call infiltrators, as well as logistics and mortar experts.

"Each one only fights for a short period and is then relieved by the next group - it exhausts the army," Hashemi said.

Although they face a coalition of Iraqi army, special forces, Kurdish peshmerga and Shi'ite paramilitary groups which may total around 100,000 fighters, the asymmetric war strategy has so far meant the 5,000-strong jihadists in Mosul have tied down the advancing troops, without using their full reserves.

Hashemi said an inner core of mainly Francophone foreign fighters, given the name 'al-Murabitoun' (Guards) had taken an oath to fight to the death defending strategic positions in the heart of the city.

"The only way they will leave is when they are dead," he said, adding they were also holding residents as human shields against air strikes.

So far the advancing forces have only breached eastern Mosul. Hashemi said two infantry divisions which have advanced close to its northern and southern limits were preparing to open two new fronts in the city, possibly as soon as Friday.

Ultimately, he said the superior numbers of the forces attacking on multiple fronts would wear down the militants. "We will win, without doubt. But it will be a costly victory".