Voters across the war-scarred nation cast their ballots under tight security, as the jihadists still pose a major security threat despite a sharp fall in violence.

The poll comes with tensions surging between key powers Iran and the United States after Washington pulled out of a key 2015 nuclear deal, sparking fears of a destabilising power struggle in Iraq.

Roughly 24.5 million voters face a fragmented political landscape five months after IS were ousted, with the dominant Shiites split, the Kurds in disarray and Sunnis sidelined.

Over 15 blood-sodden years since the US-led ouster of Saddam Hussein, disillusionment is widespread and politics is dominated by old faces from an elite seen as mired in corruption and sectarianism.

At a polling station in the Baghdad district of Karrada, 74-year-old Sami Wadi appealed for change "to save the country".

"I call on all Iraqis to participate in the elections to prevent those who have controlled the nation since 2003 from staying in power," the retiree told AFP.

In the former IS bastion, second city Mosul -- still partly in ruins from the months-long fight to oust the group -- residents hoped for an uptick in their fortunes as they struggle to put their lives back together.

"I am voting for security and the economy to stabilise and for a better future," said labourer Ali Fahmi, 26.

- 'Terror' defeated -

Prime Minister Haider al-Abadi -- who took office as IS rampaged across Iraq in 2014 -- is angling for a new term, claiming credit for defeating the jihadists and seeing off a Kurdish push for independence.

But competition from within his Shiite community, the majority group dominating Iraqi politics, will likely splinter the vote and spell lengthy horse-trading to form any government.

"Iraq is strong and unified after defeating terror," Abadi said after voting. "The elections will determine Iraq's future."

Whoever emerges as premier will face the mammoth task of rebuilding a country left shattered by the battle against IS -- with donors already pledging $30 billion (25 billion euros).

More than two million people remain internally displaced and IS -- which has threatened the polls -- is still able to launch deadly attacks.

Iraq has long been a crucible for the rivalry between Iran and the US, with Tehran exerting influence over Shiite politicians and Washington deploying troops to fight IS.

- Shiite contenders -

Overall, just under 7,000 candidates are standing and Iraq's complex system means no single bloc is likely to get anything near a majority in the 329-seat parliament.

Abadi -- a consensus figure who has balanced the US and Iran -- is facing two leading challengers to his Victory Alliance.

Ex-premier Nuri al-Maliki is widely reviled for stirring sectarianism and losing territory to IS, but draws support from hardliners.

"I wish for all to go to the ballot boxes to make their choice," Maliki said after casting his ballot, demanding authorities stop "attempts at falsification through the pressuring of voters".

Hadi al-Ameri -- a contender who led Iran-backed paramilitary units that fought IS alongside Baghdad's troops -- called for "change" as he seeks to turn battlefield wins into political gains.

Votes in Sunni heartlands once dominated by IS -- including Iraq's devastated second city Mosul -- are up in the air as traditional alliances have been shredded by the fallout of jihadist rule.

Political forces in the Kurdish community -- often seen as kingmakers -- are also in disarray after a September vote for independence spectacularly backfired.

The Kurds look set to lose some of their clout on the national stage after Baghdad unleashed a battery of sanctions and seized back disputed oil-rich regions.

Putting on a brave face, the prime minister of autonomous Kurdistan, Nechirvan Barzani, insisted the political process would not succeed "without Kurdish participation".

"No party can form the next government without alliances," he said in televised comments after voting.

A senior security official told AFP that some 900,000 police and soldiers are on high alert to protect the vote, with airports and borders shut for the day.

Polling stations are open until 6:00 pm (1500 GMT) and initial results are expected in three days.