Iraq's victory over the Islamic State group in Tal Afar was the latest in a string of gains against the jihadist group, but Iraqi forces still face massive challenges, experts say.

In 2014, IS staged a rapid advance across northern Iraq, with police and military personnel abandoning their posts to the jihadists with barely a fight.

That allowed IS to seize nearly a third of the country's territory including Mosul, which was fully recaptured by Iraqi forces in July, three years after IS declared its self-styled "caliphate" from a mosque in Iraq's second city.

Today, Prime Minister Haider al-Abadi, who took office three months after the 2014 military debacle, says the Iraqi state is back -- stronger and better organised.

Under the Shiite premier's command and backed by a US-led multinational coalition, Iraqi forces have retaken Tikrit, Ramadi, Fallujah and, after a gruelling nine-month battle, Mosul.

On Thursday, Abadi announced the recapture of the town of Tal Afar and surrounding areas, bringing the whole of Nineveh province of which Mosul is the capital under government control.

With support from foreign military trainers, Iraqi forces have "gained a lot of combat and coordination capacity in just a year", said a French military official, asking not to be named.

"Relatively well-organised and trained but also well-supported, (they) have been able to stop the jihadists organising along coherent lines of defence," he told AFP.

During the fight for Mosul, described by an American general in Baghdad as "the toughest urban battle since World War II", Iraqi troops suffered heavy losses.

But they have now forced IS out of all its Iraqi territory except the town of Hawija, 300 kilometres (190 miles) north of Baghdad, and a few pockets of territory near the border with Syria.

In doing so, they have regained "the confidence of their fellow citizens and internationally", said Jassem Hanoun, an Iraqi military expert, after the loss of face of three years ago.

- 'Not the end' -

Coalition officials say Iraqi-led decision-making and better sharing of intelligence between Baghdad and the US-led coalition have allowed for quicker, more targeted attacks.

But Iraq's Foreign Minister Ibrahim al-Jafari warned on August 26 that "victory in Iraq will not mean an end to the danger posed by IS".

He said Iraq would continue its military cooperation with the coalition, saying it needed "preventive security" against "terrorist cells working in the shadows".

Hanoun said IS would likely go back to its "original mode of operation", attacking targets such as residential districts and markets.

But a lack of coordination and organisation means the security services struggle to cope with such attacks, he said.

The question of whether and how the coalition will continue to operate in Iraq is a hot political topic both for Baghdad and for Washington, which in 2011 finally withdrew its troops eight years after leading an invasion in 2003.

Abadi's cooperation with the US poses a pressing dilemma: what will become of the Hashed al-Shaabi paramilitary coalition, key to the fight against IS but dominated by Shiite militias backed by Iran?

Most Shiite leaders call for the Hashed, currently under the command of the prime minister, to remain in its current form.

But paramilitary groups have played a problematic role in Iraqi politics as far back as the 1930s, according to Mohammad-Mahmoud Ould Mohamedou, professor of international history at the Graduate Institute in Geneva.

The Hashed "is only the most recent version of a national politico-security configuration that has been combined with a sectarian component since 2003", he said.

The Iraq specialist said the Hashed's existence was an "admission of the failure of an army trained by US administrations at great financial and material cost over 14 years".

Alleged abuses both by government forces and Hashed fighters during the fight against IS will complicate efforts to regain the confidence of Iraq's Sunni minority, marginalised and out of power since the 2003 fall of dictator Saddam Hussein.

On top of the sectarian question, Iraq faces another challenge to its coherence as a nation: a referendum on independence for its autonomous Kurdish region, set for September 25.

The US and coalition members are strongly opposed to the poll, saying it could distract from the fight against IS.

Baghdad is also set to hold provincial and parliamentary elections in spring 2018, posing a test for Abadi.

The premier has "made the success of the military campaign a selling point as a way to prove his 'reforms' are working", said Kirk Sowell, a political risk analyst and publisher of Inside Iraqi Politics.

But Abadi's campaign could suffer from low oil prices, which have hit the Iraqi state's coffers hard.

"As the election will not be held sooner than next April or May, by then the lousy economy could weigh more heavily on voters' minds" than this summer's military victories, Sowell said.