The shortcomings of the Iraqi military, which withered under a June militant onslaught and relies on Shiite militias for support, pose a significant challenge to international efforts against brutal jihadists.

President Barack Obama has announced an expanded campaign of American air strikes as well as training, intelligence support and equipment for Iraqi security forces to help them drive back the Islamic State (IS) jihadist group, which holds major areas in both Iraq and Syria.

And Washington is seeking to build an international coalition against IS, bringing countries on board to provide military assistance, humanitarian aid, target the group's financing and work to stem the flow of foreign fighters.

But in Iraq, police and soldiers lost control of all of one city and part of another west of Baghdad early this year, multiple Iraqi divisions folded when faced by an IS-led offensive in June and aside from small elite units, security forces have since struggled to push the militants back.

It will be up to these forces to retake and hold ground.

Deep Sunni Arab mistrust of the Iraqi government will also cause difficulties for anti-jihadist efforts, while long-running Arab-Kurd disputes may also be a problem.

The army "was not a force that was combat ready at the end of 2011," when US troops exited Iraq, said Anthony Cordesman, an expert on the country's security forces from the Center for Strategic and International Studies.

Former premier Nuri al-Maliki then "bled down the quality of the force, corrupted it, made it loyal to him," Cordesman said, adding that "it doesn't take that long to politicise and destroy a force which wasn't ready in the first place."

- 'Little training, limited experience' -

"The Iraqi security forces could use a lot more training and experience," said John Drake, a security analyst with risk consultancy AKE Group.

"Between 2011 and the beginning of 2014 they went through a period of little training and limited battle experience," Drake said.

"Some units were engaged in counter-insurgency operations but large parts of the force spent most of their time maintaining checkpoints and checking vehicles, rather than actively being involved in combat operations against militants."

US Secretary of State John Kerry said during a visit to Baghdad on Wednesday that the Iraqi army "will be reconstituted and trained and worked on".

Washington has already spent billions of dollars training and equipping Iraqi security forces.

In the months since the start of the militant offensive in June, Baghdad's forces have launched multiple unsuccessful attempts to retake the city of Tikrit and have largely struggled to regain ground.

Their biggest successes have so far come with heavy involvement of Shiite militias, which have a history of brutal sectarian killings and include members who fought American forces in past years.

"Over the short term, almost all of the offensive missions will include some form of Shiite militia participation and volunteers under the leadership of the Iraqi military," said Ayham Kamel, Middle East and North Africa director for the Eurasia Group consultancy.

But over time, "US support for the Iraqi army will strengthen its hand and decrease the need for support from volunteers and militias," he said.

- Regaining Sunni trust -

Iraqi reliance on Shiite militia forces risks further alienating the Sunni Arab minority, the support of which will be key in regaining mainly-Sunni areas that IS holds.

Many Iraqi Sunnis are deeply mistrustful of the Shiite-led government, viewing it as hostile to their community and politicians and having targeted them with the security forces.

Experts say Sunni anger played a significant role in the revitalisation of militant groups including IS, decreasing cooperation with the government, easing recruiting and giving them more freedom to operate.

Prime Minister Haidar al-Abadi has proposed forming a "national guard" under which Iraq's various provinces would be defended by local forces, a plan that would put Sunnis in charge of security in Sunni areas and could help regain their trust.

Obama has backed the effort, but it could face difficulties due to the Iraqi government's past poor treatment of Sunni tribesmen who fought militants alongside the US.

Long-standing differences between the country's Arabs and Kurds, who have a three-province autonomous region in the north, could also pose problems for coordination between federal forces and Kurdish fighters who are battling jihadists in the north and east.

Kurdish forces also moved into disputed northern territory when federal forces withdrew earlier this year.

While the Kurds want to incorporate these areas into their region, Baghdad wants them back -- an issue that will come to the fore if federal forces later succeed in advancing north.