Iraqi forces launched an offensive on jihadists defending Mosul's west bank Sunday, in what could be the most brutal fighting yet in a four-month-old operation on the country's second city.

They swiftly retook two villages and set their sights on Mosul airport, which lies just south of the city, marking a new phase in the offensive, Iraq's largest military operation in years.

The Islamic State group has put up stiff resistance to defend Mosul, the city where its leader Abu Bakr al-Baghdadi proclaimed a "caliphate" in 2014.

"Our forces are beginning the liberation of the citizens from the terror of Daesh," Iraqi Prime Minister Haider al-Abadi said in a short televised speech, using an Arabic acronym for IS.

"We announce the start of a new phase in the operation. We are coming, Nineveh, to liberate the western side of Mosul," he said, referring to the province of which Mosul is the capital.

A top army commander then announced that forces led by federal police units retook Athbah and Al-Lazzagah, two villages on the southern approach to Mosul that lie on the east bank of the Tigris River.

An AFP reporter near the front line said heavy bombardment could be heard as forces backed by Iraqi and coalition aircraft advanced towards the jihadists' last major bastion in the country.

The jihadists overran Mosul and swathes of other territory north and west of Baghdad in 2014, sweeping aside security forces ill-prepared to face the assault.

The Iraqi government launched the offensive to reconquer Mosul on October 17, throwing tens of thousands of forces into the long-awaited counter-attack with air and ground support from the US-led coalition.

The Joint Operations Command coordinating the fight against IS declared east Mosul "fully liberated" on January 24.

But it took Iraq's most seasoned forces -- the elite Counter-Terrorism Service -- more than two months to clear the eastern side of Mosul.

After a pause, federal forces now face what was always billed as the toughest nut to crack: Mosul's west bank, home to the narrow streets of the Old City.

"West Mosul had the potential certainly of being more difficult, with house-to-house fighting on a larger and more bloody scale," said Patrick Skinner, from the Soufan Group intelligence consultancy.

- Tougher resistance -

The streets around the historical centre, which includes the mosque in which Baghdadi made his only public appearance in June 2014, will be impassable for many military vehicles and force government fighters to take on IS in perilous dismounted warfare.

Prior to the offensive that saw IS seize Mosul and much of Iraq's Sunni Arab heartland nearly three years ago, the east bank was more ethnically diverse than the west, where analysts believe the jihadists could enjoy more support.

"IS resistance could be greater in this area and it will be harder, but all the more important, to completely clear the networks from Mosul after its recapture," said Emily Anagnostos, Iraq analyst at the Institute for the Study of War.

While the federal forces' attrition is said to be high, IS's had been undoubtedly higher and commanders have said the jihadists may no longer have the resources to defend east Mosul effectively.

Recent incidents in the recaptured east point to the difficulty of ensuring remnants of IS have not blended in with the civilian population in a huge city which most residents did not flee ahead of the government offensive.

Aid organisations had feared an exodus of unprecedented proportions before the start of the Mosul operation but half a million -- a significant majority -- of residents stayed home.

- Trapped civilians -

Their continued presence prevented both sides from resorting to deadlier weaponry, which may have slowed down the battle but averted a potentially much more serious humanitarian emergency in the middle of winter as well as more extensive material damage to the city.

"Mosul is going better than we expected, but there are serious dangers ahead," Lise Grande, UN humanitarian coordinator in Iraq, told AFP.

Residents of west Mosul have reported very difficult living conditions and warned that they were already low on food, with weeks of fighting expected to lie ahead.

IS fighters and Mosul residents remained able to move across both sides of the city during much of the fighting in the east but all bridges across the Tigris have now been dropped and the jihadists in the west are all but besieged.

IS has used civilians as human shields as part of its defence tactics and killed residents attempting to flee, making it both difficult and dangerous for the population to escape.

While specialised units may attempt to throw pontoon bridges across the river to attack from the east, the main initial assault of the upcoming phase in the Mosul is expected to come from the south on the city's airport.

Key dates in battle for Mosul
Baghdad (AFP) Feb 19, 2017 - Iraqi government forces launched a major offensive to recapture the city of Mosul from the Islamic State group on October 17, 2016.

Here are key dates:

- Battle for Mosul begins -

- October 17: Iraqi forces launch an offensive to drive IS out of Mosul, where the jihadists declared an Islamic "caliphate" in June 2014.

IS overran Mosul and swathes of other territory north and west of Baghdad in 2014, sweeping aside security forces ill-prepared to face the assault.

Tens of thousands of troops from army, police and counter-terrorism units are thrown into the long-awaited counter-attack with air and ground support from the US-led coalition.

By the end of October, the army has recaptured the Christian town of Qaraqosh, 15 kilometres (10 miles) from Mosul. Dozens of other nearby towns are retaken within two weeks.

- Entering Mosul -

- November 1: The army says it has entered Mosul itself for the first time since June 2014.

- November 3: IS chief Abu Bakr al-Baghdadi breaks a year-long silence to urge his fighters to defend Mosul to the death, and the advance of Iraqi forces begins to slow down.

- November 8: Kurdish peshmerga fighters say they have reached Bashiqa, a dozen kilometres (about eight miles) north of Mosul.

- November 13: Iraq says it has recaptured Nimrud, an ancient city southeast of Mosul.

- November 23: Shiite-dominated paramilitary units known as Hashed al-Shaabi say they have cut IS supply lines between Mosul and Raqa, the self-declared jihadist capital 400 kilometres (250 miles) to the west in Syria.

- Change of tactics -

- December 29: Government troops end a two-week pause by launching the second phase of their offensive on east Mosul with increased coalition support and improved coordination between fighting units.

- January 4: A US-led coalition spokesman indicates the number of Western advisers in the battle has doubled to around 450.

- Tigris River bank -

- January 8: Iraqi units reach the Tigris River that divides Mosul and take up positions near one of the city's five bridges, which have all been destroyed.

- January 14: Elite forces from the Counter-Terrorism Service take control of the sprawling campus of Mosul University.

- Getting there -

- January 18: The head of special forces announces the liberation of the east bank, two days after Iraqi forces reach the iconic Nabi Yunus shrine, also known as "Jonah's tomb" and which IS destroyed in 2014.

Sporadic fighting continues for four more days however, and the western side of Mosul, home to the Old City and traditional jihadists' bastions, is expected to offer much stiffer resistance.

- January 24: The Joint Operations Command coordinating the fight against IS says that the east has been "fully liberated", after pockets of IS fighters in the north are cleared.

- Battle for western Mosul -

- January 24: The UN warns that 750,000 civilians in western Mosul are "at extreme risk" as Iraqi forces prepare to attack IS fighters in that part of the city.

In February it adds that up to a quarter of a million Iraqis could flee their homes. That figure comes on top of the 200,000 who have fled since October 17. Close to 50,000 of them have since returned to their homes.

- February 19: Iraqi Prime Minister Haider al-Abadi announces the launch of operations to retake the western side of Mosul. The initial phase of that offensive is expected to focus on Mosul airport, just south of the city.