Identification cards and cell phones in hand, dozens of Iraqi men and boys trudge down a dusty road in eastern Mosul towards intelligence officials waiting apprehensively outside the neighbourhood mosque.

"Stop! Stop where you are!" an Iraqi police officer calls out to the group as he nervously pats down the first arrivals to check for weapons or suicide vests.

At noon on Friday, instead of weekly prayers, the men were herded into the mosque in the Karkukli district for questioning by federal police and the Counter-Terrorism Service's intelligence branch.

Iraqi forces are pressing a month-long offensive to seize the Islamic State group stronghold of Mosul.

After seizing a neighbourhood, they gather males aged 13 and older at local mosques or schools to press them for information about the jihadist group.

"If you have names, give them to us. If you have any information, we want it," the policeman shouts as the line grows longer.

About 30 men are ushered into the mosque as the others are told to crouch down in single file.

One resident is ordered to pull his trousers down to prove he is not wearing a belt of explosives. He stands with his arms extended, a cigarette hanging from his mouth.

- Texts from informants -

Once inside the mosque's courtyard, residents hand their ID cards and phones to three intelligence officers hunched over laptops.

The officials check the names against lists of suspected IS fighters, and scroll through text messages for potentially incriminating information.

"We took their papers and they will be called up by their IDs one by one," says Major Alaa Abdel Omran of the CTS's intelligence branch.

"If he doesn't have a terrorist background then he will be let go and he can go home," he tells AFP.

Most of the men are released after a few minutes, pressed by Iraqi officers on their way out to tell journalists that the questioning process is gentle and fair.

But one man is escorted from the mosque by federal police, his face covered and pulling on a sweatsuit emblazoned with the CTS logo over his own clothes.

They clutch his arms and walk away from the group, reportedly so he can lead them to a local IS official's home.

Iraqi forces say local informants have been crucial to their advance inside Mosul, particularly as they push through densely populated residential districts in the east.

Major Hazem al-Bahadli, who heads the CTS's Salaheddin Regiment, says he has developed at least three or four local sources inside each neighbourhood.

"They send us text messages about how many Daesh fighters there are in a given position, where they are planting car bombs or placing their snipers," he tells AFP, using the Arabic acronym for IS.

- 'Not walking blind' -

Bahadli shows AFP one text message warning that "Chechen fighters" were setting up sniper positions in tall buildings in the area.

He receives the phone numbers of potential informants from relatives or neighbours who have fled, or from residents of recently captured neighbourhoods with friends in adjacent districts.

"This information means we aren't walking through these neighbourhoods blind -- we know exactly where the enemy is," he says.

IS supporters have sometimes infiltrated the informant network to spread rumours, like the threat of a chemical attack.

When his forces seize a neighbourhood, Bahadli gets the chance to meet face-to-face with informants he had only known previously from their text messages.

"He'll come ask after me because he knows me by name. My job, then, is to help him out just like he helped me," Bahadli says.

But if they are caught helping Iraqi forces, Mosul residents pay with their lives, according to Hussameddin al-Abbar, a member of the Nineveh provincial council.

"IS executed nearly 60 people several days ago and hung their corpses on electricity poles in the city's west after accusing them of working with security forces," he told AFP earlier this week.