The materials are used to make primitive powerbanks to charge the walkie-talkies that SDF commanders rely on to communicate with each other across Raqa's frontlines.

In a cavernous warehouse just east of Raqa's Old City, SDF fighters sit cross-legged on a dusty rug piled high with cylinder-shaped three-volt batteries, masking tape, empty cigarette packs and loose wires.

But winning their months-long offensive against the jihadists, they say, required going back to basics.

The Syrian Democratic Forces on the verge of seizing IS's former bastion Raqa have received sophisticated support from the US-led coalition, including air strikes, weaponry, and intelligence.

Once the last Islamic State group fighters are ousted from Syria's Raqa, the unconventional forces battling the jihadists say they'll have batteries and masking tape to thank for their victory.

As artillery fire and air strikes echo in the background, the assembly line gets to work. One fighter stacks eight batteries into a brick-like shape while another prepares the tape that will hold them together.

A third peels the aluminium foil from white cigarette packs -- perfect for a conductor -- and begins taping it to the wires he snipped from the walls of the battered building.

"Our positions have to be in touch 24 hours per day with

these walkie-talkies, but they're not that great," says local SDF commander Sevger Himo.

- 'Mother of invention' -

Built-in batteries last just three hours, which forced SDF commanders to switch them off between coordinating storming operations, defensive rocket fire, or civilian rescues.

But with the hand-made battery packs, walkie-talkies stay charged for up to two straight days.

The powerbanks are ubiquitous in battle-ravaged Raqa, including in frontline positions near the city's hospital and football stadiums, two of IS's last redoubts in the city.

Members of the Syrian Democratic Forces fire their weapons during a battle against Islamic State group jihadists in Raqa on September 28 (photo by: BULENT KILIC/AFP)

SDF fighters advancing there are often isolated for days from rear bases, making fully-charged walkie-talkies their only link to the outside world.

"This has saved lives more than once. Necessity is the mother of invention, as they say," says Himo, a young fighter with a boyish face.

"National armies have their own factories, but we're a military force without much international support -- so we rely on things we can get in the market and then adjust."

Founded in late 2015, the SDF is not a conventional army and many of its young Arab and Kurdish fighters have received no previous military training.

In Raqa, they fight in dusty sneakers or even plastic sandals and sustain themselves on a diet of water, bread, and extremely sugary tea.

SDF forces have even developed creative tripwire systems in the multi-storey apartments they are using as bases near the frontline.

Crushed plastic water bottles are scattered all along the tiled staircase in one apartment building near the national hospital -- but SDF commander Gabar Derek insists they are not litter.

"If a stranger tries to come up the stairs at night, he will step on these bottles and make noise, warning the fighters on watch duty upstairs," says the bearded fighter.

He says he used a cowbell tied to a wire in another position

for the same purpose.

'Hand-made, locally produced'

One of the SDF's favourite innovations, however, remains a hand-made explosive device dubbed "the fuse".

Each melon-sized, misshapen package is filled with TNT, held together by tape and features a six to eight-inch (15 to 20 centimetre) fuse and a long fabric handle.

"We use the handle so we can carry four or five at a time," says Mohammad, a boisterous fighter positioned in the central neighbourhood of Al-Nahda.

is the perfect weapon against Islamic State group tunnels, one of which is shown here in a photo from the US State Department (photo by: HO/US Department of Defense/AFP)

Indeed, he has four "fuses" hanging around his neck and says proudly that they are "hand-made and locally produced" -- though the SDF refused several requests by AFP to see the factory on the outskirts of Raqa where they are made.

Mohammad says the bombs are the perfect weapon to counter IS's toughest tactics: snipers and secret tunnels.

"We toss these in streets monitored by snipers, and they bring up a lot of dust so we can cross safely without snipers seeing us," he says.

More conventional explosives are less effective because they don't create as opaque a smokescreen or as loud a blast.

"If there's a bunker or tunnel used by Daesh, we just throw one inside. Or two, if that's not enough," Mohammad says, using the Arabic acronym for IS.

He cheerfully sets off into the rubble-littered street with a "fuse" and a lighter to support a fresh push by his forces.

"This saves lives," he says.