Iraqi forces are fighting a muddy street battle against jihadists in southeastern Mosul, facing suicide bombers on the ground and a drone that can drop explosive charges from above.

Baghdad's forces have overwhelming firepower and numbers in Mosul, but the Islamic State group has a vast city in which to launch ambushes, plant bombs and try to make the battle as slow and costly as possible.

More than two months into the operation to retake the city, Iraqi forces have recaptured a large chunk of east Mosul, but IS still holds parts of it as well as all of its western side.

In southeastern Mosul, a small explosion sounds behind the front line, sending a member of Iraq's elite Rapid Response Division falling to the mud-covered ground, his legs wounded by shrapnel.

Security forces members train assault rifles and machineguns skyward, unleashing a barrage of fire at the small white drone that apparently dropped the explosive device.

IS jihadists "are flying an observation aircraft that carries a bomb or a grenade," Lieutenant Colonel Hisham Abdulkadhim, the commander of a regiment in the Rapid Response Division, explains.

"Where it see units, it drops it," Abdulkadhim says.

Members of the Rapid Response forces swathe one of their wounded comrade's legs in bandages, then cut away the bloodied fatigues from the other, treating it as well.

The wounded man lies stoically on a black stretcher as they work, leaving behind a trail of blood when he is carried to an armoured Humvee vehicle for evacuation.

A white drone later returns, hovering far overhead.

"Don't gather together," warns a member of the federal police, which are fighting alongside Rapid Response Division.

Iraqi forces also have their own drone, which is periodically sent aloft to scout the path ahead.

On the ground, there are far more dangerous threats than the IS drone above: a suicide bomber driving an explosives-rigged vehicle is spotted heading toward Iraqi forces.

Members of the security forces sprint for cover behind a dirt barricade and a Humvee mounted with an anti-tank missile launcher prepares to fire.

"A car bomb tried to get close. We withdrew the units that were covering the front so that the Kornet (missile) system could deal with" it, Abdulkadhim says.

- Divide, clear, advance -

But the suicide bomber apparently pulls back, and the missile is not fired.

The berm behind which security forces took cover is part of a strategy of dividing and then clearing an area as they advance.

"We divide the area into squares," Abdulkadhim says.

They then clear houses, put in place snipers and weapons to deal with car bombs, and move on.

A bulldozer painted in a blue and grey camouflage pattern slowly moves through the thick mud that coats the street after recent rain, then scoops up pile after pile of dirt until the street is blocked, protecting forces behind it from suicide bombers prowling the city.

When the forces are finished clearing the area, the bulldozer forces a pathway through the berm, moves farther down the street, erects a new barrier, and the process is repeated.

This makes for slow going, but provides extra security.

Iraqi forces are backed by US-led air strikes and Iraqi aviation units, as well as artillery fire from the ground.

Throughout the day, massive rockets shriek over Mosul, sending clouds of dust rising in the distance when they explode.

Three of the rockets are mounted on the back of a truck in launch tubes that are raised to point them in the direction of their targets.

Security personnel fit fuzes to the tips of the rockets and wires to the other end to ignite them.

People clear the area around the truck, mobile phones come out to record, and the rocket is launched, disappearing over the rooftops trailing a tail of flame.

"Wherever there is a gathering of (IS) terrorist gangs, they give us information about them," says Jassem Abbas, who helped ready the rocket for launch.

Then "according to the coordinates, we bomb it."

Iraq forces launch new assault on IS in Mosul
Baghdad (AFP) Dec 29, 2016 - Iraqi forces advanced Thursday after declaring a new phase in their offensive on eastern Mosul, stepping up efforts to reclaim the Islamic State group's last major stronghold in the country.

Elite forces have reconquered several parts of eastern Mosul since beginning the massive operation to recapture the northern city from the jihadists on October 17, but IS still occupies the city's west.

Security chiefs said several neighbourhoods had been taken, with the bodies of IS fighters seen lying on the streets and drone footage showing others retreating.

Retaking Mosul could effectively end the jihadist group's days as a land-holding force in Iraq and deal a death blow to the "caliphate" IS leader Abu Bakr al-Baghdadi proclaimed in the city in June 2014.

"The second phase of liberating the left bank in Mosul was launched, and our forces began advancing toward Al-Quds neighbourhood," said Staff Lieutenant General Abdulghani al-Assadi, a senior officer in Iraq's Counter-Terrorism Service.

"Our forces clashed with the enemy and there is resistance," he told AFP, adding forces on the northern and southern fronts were also advancing.

Mosul, now the last Iraqi city in which IS still holds significant territory, is split by the Tigris River, with the east side referred to as the left bank and the west as the right.

Iraq's federal police commander, Raed Shaker Jawdat, said "the enemy's line of defence has fallen back" inside the city.

"The streets of the Al-Salam, Al-Intisar, Al-Wahda, Al-Falestin and Al-Quds neighbourhoods are strewn with the bodies of Daesh fighters," he said, using an Arabic acronym for IS.

Police had also entered deep into the district of Jadida al-Mufti.

The commanding chief of the offensive, Staff Lieutenant General Abdulamir Yarallah, late Thursday announced "the liberation of the Al-Quds 1 district".

- US probes 'civilian casualties' -

The commander said police drones had spotted IS fighters retreating to the city's western side via a pedestrian bridge badly damaged in the fighting.

Centcom, the US military command in the Middle East, said the US-led coalition fighting IS had disabled "the last bridge crossing the Tigris river inside Mosul" this week at the Iraqi government's request.

It said meanwhile that an investigation had been opened after a US-led air raid on an IS van during the Mosul operations on Thursday struck "what was later determined to be a hospital compound parking lot resulting in possible civilian casualties".

A Mosul inhabitant who requested anonymity reported hearing many explosions and said residents were holed up indoors.

The new push in the battle for Mosul comes after progress slowed to a crawl in the past few weeks.

After two months of fighting, Iraqi Prime Minister Haider al-Abadi last week said the army had decided to review battle plans and pause to reduce losses.

The fighting has been all the more devastating as it has been happening in a city amid civilians.

IS has targeted the army with car bombs on at least 900 occasions in the streets of Mosul since October 17, according to Abadi.

IS overran large areas north and west of Baghdad in 2014, but Iraqi forces backed by US-led air strikes have since regained much of the territory they lost to the jihadists.

Abadi had pledged that Mosul would be recaptured by year's end -- a goal that is now out of reach, as operations in western Mosul have yet to begin.

The Iraqi prime minister also said this week that three months were needed to eliminate IS in the country.

High-ranking officials believe the battle against IS could drag on, and the Sunni Muslim extremist group continues to carry out attacks in areas from which it had been dislodged by the government offensive.

Last week, an IS triple car bombing on a market in Gogjali a few kilometres (miles) east of Mosul killed at least 23 people.

The army had retaken Gogjali from the jihadists in early November after more than two years of IS occupation.