Mosul (Iraq) (AFP) - "Two grooms are coming," a low voice crackles over the walkie-talkie, prompting a lanky Iraqi special forces soldier listening in Mosul to shout: "Boys! They've just dispatched two suicide bombers!"

The small walkie-talkie that Iraq's elite Counter-Terrorism Service seized from an Islamic State jihadist group member in Mosul last week has proven priceless in their drive for Iraq's second city.

All day, members of the CTS's Mosul and Najaf regiments take turns clutching the device up to their ears to intercept communications between jihadists in the city, the last one IS holds in Iraq.

They try to decipher IS codes and pick up where exactly jihadists are positioned in nearby neighbourhoods.

"Last week, we saw a man transporting some goods on his motorcycle in this neighbourhood, and we suspected him of being an IS member," said Staff Lieutenant Colonel Muntadhar Salem, head of the CTS's Mosul regiment.

He raised his voice to a near-yell so he could be heard over the incoming and outgoing mortar fire in Mosul's eastern Al-Samah neighbourhood as he related how the walkie-talkie was obtained.

"I saw he had this walkie-talkie clipped to his shirt, so we took it from him and kept it," Salem said, refusing to elaborate on what happened to the man, whose name was apparently Abu Yusef.

"Someone on the other end was calling 'Abu Yusef, Abu Yusef,' and Abu Yusef wasn't answering," he chuckled.

The battle to retake Mosul is now in its fourth week, and CTS forces have been at the forefront of the assault on the city's east, pushing IS back from several neighbourhoods.

But there are still weeks, if not months, of fighting still to go.

- Keep tabs on IS moves -

Five IS walkie-talkies have been seized so far in Mosul and divvied up among various CTS regiments so that each can keep constant tabs on IS movements.

The Najaf regiment was the unit that figured out references to grooms indicated incoming suicide attackers.

"It's because they believe they'll go to heaven and marry many women," scoffed Staff Lieutenant Colonel Ali Fadhel, commander of the regiment which has been monitoring Abu Yusef's walkie-talkie this week.

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"They haven't changed their channels, which means they haven't figured out that we're listening," Fadhel said.

Two of his fighters -- Ahmed and Mohammed -- are on walkie-talkie duty.

Brows furrowed, the device sandwiched between them, they crouch outside the two-storey house that CTS forces have made their base in Al-Samah.

"That was an accent from Mosul, but sometimes you hear Algerian or Moroccan accents," Mohammed said.

A voice speaks up from the walkie-talkie: "Hussam? Hussam?"

Ahmed rolls his eyes, explaining that much of his time listening is spent waiting for valuable information or trying to make sure he doesn't lose the signal.

"But sometimes they reveal which neighbourhoods they're in and which weapons they're going to use against us -- whether mortars, rockets, snipers, or car bombs," he said.

Ahmed -- who first warned about the two incoming suicide bombers -- called out to his comrades just a hundred meters (yards) ahead, giving the position of IS fighters.

"Al-Qahira! They're between Al-Qahira and Tahrir!"

CTS members started firing mortar rounds towards the Tahrir area as Ahmed pressed the walkie-talkie back up to his ear once more.