Two years after they withdrew from Mosul broken and defeated, Iraqi forces have capped a dramatic turnaround by reaching the largest city they lost to the Islamic State group.

Just over two weeks into a massive offensive to retake Mosul from IS, Iraqi army and special forces personnel have reached its outskirts for the first time since 2014.

It is the culmination of a long effort to retrain the Iraqi armed forces and replace incompetent commanders that has seen them regain much of the territory lost when IS swept across the country in June 2014.

"The army is not the same army," said Staff Lieutenant General Qassem al-Maliki, the commander of the 9th Armoured Division, one of the units that has forces near Mosul.

Some Iraqi soldiers and police who fought in Mosul in 2014 are taking part in the operation to retake it now, while many others are veterans of previous battles against IS.

The US-led coalition against IS began training Iraqi forces in the wake of the initial jihadist onslaught, and has provided essential air and artillery support as well as advice to the country's troops.

Coalition spokesman Colonel John Dorrian said that 54,000 Iraqi soldiers, police and Kurdish peshmerga fighters have gone through the training programme so far.

Iraqi Prime Minister Haider al-Abadi has also sacked a number of Iraqi military officers since taking office in 2014, clearing the way for more competent commanders to take command.

Returning to Mosul "is a symbolic moment for (Iraqi forces), who had collapsed in spectacular fashion in June 2014 but return with coalition backing and new training," said Patrick Martin, an Iraq analyst at the Institute for the Study of War.

Iraqi forces went on the offensive beginning in 2014, retaking an area south of Baghdad, and have slowly pushed north and west of the capital, regaining a series of IS-held cities and towns.

- 'High-quality leaders' -

"The Iraqi army has won every battle it has fought since May 2015," said Michael Knights, a fellow at the Washington Institute for Near East Policy.

"The key driver has been the high-quality leaders that now command the key Iraqi units, almost all drawn from the best combat commanders active in 2007-2011," before then-premier Nuri al-Maliki purged the military, Knights said.

There is likely still weeks if not months of heavy fighting ahead for Mosul.

But reaching the city is a major accomplishment in and of itself, especially given where Iraqi forces were in 2014.

Some Iraqi soldiers and police fought IS when it attacked Mosul that year, but forces in the city ultimately failed to hold off the attack despite outnumbering the jihadists, with some breaking and running while others withdrew under orders.

It was an unmitigated disaster for the country, which lost its second city while at the same time leaving a wealth of vehicles and other equipment that IS has since used against Iraqi troops.

A parliamentary inquiry into the debacle found that top Iraqi officials ignored ample warnings of the impending attack, and that "the only surprise was the speed with which the military units collapsed."

Top officers performed poorly once the assault began, while Maliki had already fundamentally weakened the military by appointing "incompetent leaders and commanders," the inquiry found.

Martin of the Institute for the Study of War cautioned that problems remain within the Iraqi security forces, including "manpower shortages and corruption at multiple levels".

Still, retaking the city where it suffered one of its worst defeats would "be a critical capstone in the recovery of the Iraqi army," Knights said.