Arbid (Iraq) (AFP) - The Islamic State group drone hovered in the sky over the advancing Iraqi forces before dropping a grenade, the jihadists' latest move to weaponise small off-the-shelf aircraft.

Down below, the grenade exploded on the roof of a building where Iraqi police forces were sheltering as they advanced some 10 kilometres (six miles) south of Mosul, the last IS-held Iraqi city.

No one was injured, according to an Iraqi officer, but the incident nonetheless represents another escalation in the war of commercially available drones that is playing out as Iraqi forces battle the jihadists.

Masters of invention, IS jihadists have booby trapped household appliances and turned cars into armoured suicide bombs as they try to stymy the Iraqi forces.

Now they seem to have found another way to try to slow the progress: weaponising the $1,000 drones that they normally use to spy on their foes.

"We have recorded three incidents," police Lieutenant Colonel Hussein Moayyad told AFP.

The jihadists appear to have used an add-on -- similar to those intended to help fisherman drop their hooks farther out at sea -- to release the drone's payload, Moayyad said.

They rig the grenade so the pin is pulled free when the explosive device is dropped, arming it.

While this attack was relatively primitive and -- for now -- pretty ineffective, IS drones have already proved more deadly in other ways.

Last month a hobby plane rigged with explosives killed two Iraqi Kurdish peshmerga fighters and injured two French soldiers.

According to a US defence official, the incident unfolded on October 2 when a small plane with a styrofoam body was either shot down or crashed in Arbil in northern Iraq.

Two Iraqi Kurdish peshmerga fighters grabbed it and took it back to their camp to inspect and photograph it, when it blew up.

- Duelling drones -

IS is flying drones to spy on Iraqi forces -- so Iraqi forces are sending up their own devices to spot the enemy as well.

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Moayyad watched a screen inside a specially converted armoured bank van he has turned into a mobile drone control centre.

"Now I am entering the dangerous zone, this is where Daesh is," he said, using an Arabic acronym for IS as he manoeuvered the drone's remote control to focus on jihadist positions some five kilometres (three miles) away.

Like the IS operation, the Iraq police have also cobbled their drone programme together with shop-bought equipment and ingenuity.

Moayyad -- who has a masters degree in computing -- modified drones bought in Dubai and Turkey to give them greater range, longer battery life and the ability to film at night.

When he spots enemy movement, he coordinates with the Iraqi artillery, air force or sometimes the US-led coalition bombarding IS from the sky.

In eastern Mosul, Iraqi special forces soldiers are using drones for the same purpose.

"There were three car bombs coming out from Al-Bakr toward our positions that we spotted with our drone and hit with our tanks," Staff Lieutenant Colonel Muntadhar Salem recently told AFP, referring to an area in the city.

In total, Moayyad said, the Iraqi police force drones -- superior to the ones IS use -- end up costing somewhere around $26,000.

But despite having superiority over the jihadists, he said the Iraqi forces could do with equipment that can let you take control of unknown drones, especially now that IS is arming them.

"Maybe they could get bigger drones," Moayyad said.

"And if they manage to use chemical weapons on them, then this is more scary, of course."