MOSUL, Iraq (Reuters) - The families cowered in basements, huddling in the dark as war raged overhead between Iraqi forces and Islamic State militants fighting for control of the streets of Mosul.

Above ground, soldiers from Iraq’s Rapid Response division move from house to house through the same openings Islamic State militants smashed through the walls in preparation to defend their last remaining stronghold in the city’s west.

The passageway led them through living rooms and gardens, into a kitchen with a pot of lentil soup on the counter -- the scenes of domesticity highlighting the chaos of war that is intensifying as Iraqi forces advance.

“It’s strange and terrifying,” said a young woman who was barely visible in the gloom of a basement under her house in the Josaq district, where she went into hiding after giving birth to a baby girl 72 days ago. “I rarely go upstairs.”

Iraqi forces advanced quickly in the early stages of the offensive to recapture Mosul’s western half, retaking the airport and piercing Islamic State defenses around the city within days.

Now they are encountering tougher resistance as they push into residential districts where as many as 750,000 civilians are essentially trapped.

If they defeat Islamic State in Mosul that would crush the Iraq wing of the caliphate its leader Abu Bakr al-Baghdadi declared in 2014 over parts of Iraq and neighboring Syria.

FOREIGN FIGHTERS BATTLE TO THE END

The Iraqi soldiers climbed through a hole in the wall of a garden strewn with ripe oranges and shattered glass, and emerged to find an Islamic State fighter lying flat on his back where he had been killed by Iraqi forces.

An Iraqi Special Forces soldier moves through a hole as he searches for Islamic State fighters in Mosul, Iraq February 27, 2017 REUTERS/Goran Tomasevic

Lt. Colonel Abdel Amir al-Mohammadawi went through the dead man’s pockets after making sure he was not wearing a suicide belt, and pulled out a small address book containing telephone numbers of other fighters, and a contact for “Islamic Police”.

“He’s not Iraqi. He’s probably not an Arab,” said Mohammadawi, judging the man by his appearance and imperfect spelling. “The closer we get to the center, the more we come up against the foreigners”.

Unlike Iraqi militants who can blend in with civilians and possibly slip through the net of security forces, foreign fighters have no escape and will therefore fight to the end. Mohammadawi said: “They don’t flee like the locals”.

There are noticeably more foreign militants in the western half of the city than the east, which Iraqi forces cleared one month ago after 100 days of fighting, Mohammadawi added.

After losing the east, Islamic State militants prepared for battle in the west, knocking holes through the walls and expelling residents whose homes offered a vantage from which to fire at advancing Iraqi forces.

At one point, the passage led into an empty hall where a motorcycle was parked. Evidently it had been used by the militants because there was a prayer mat in the plastic crate attached to the back, the soldiers said.

“Search upstairs!,” Mohammadawi ordered, sending two men up the stairs, gun barrels first, to make sure no militants were hiding there.

Also found were paper slips granting Islamic State members leave for short periods of no longer than a day, which one officer said indicated they had no time or manpower to spare.

CIVILIANS HIDE IN BASEMENTS

The densely populated terrain is already proving a challenge. Mohammadawi said Rapid Response forces had been forced to pause their advance in Josaq on Sunday because five Islamic State snipers were hiding among civilians.

A tactical unit had then killed the militants in an overnight raid, Mohammadawi said, clearing the way for Rapid Response forces to reach the first of five bridges that straddle the River Tigris bisecting Mosul.

As the sounds of artillery and small arms fire reverberated, a group of civilians came running across the street toward Iraqi forces, the women weeping in fear. The soldiers corralled them into a house where the women went down to the basement.

The women described how the militants had set the upper floors of their homes ablaze to create a smokescreen against coalition aircraft.

Mahmoud, who was amongst the group, escaped after secretly contacting the commander in the area, keeping his phone on silent so the ring tone would not give him away to Islamic State militants. But his brother stayed behind.

“Brother, get out,” Mahmoud said urgently over the phone to his brother. “It’s better. Get your stuff together. Protect your children. No inch is safe.”