Twelve people were killed in Baghdad on Thursday, including seven soldiers and police blown up by suicide bombers, days before a poll that will test Iraq's prospects for stability as U.S. troops prepare to leave.

Thirty-five soldiers and police were also wounded when two attackers with explosive belts struck at centres where security forces were voting early, an Interior Ministry source said.

A powerful explosion earlier killed five civilians and wounded 22 in Baghdad's northwestern district of Hurriya.

The assailants seemed bent on disrupting the special voting by troops, police, detainees and the sick, hoping to undermine the government and deter voters from turning out on Sunday.

The election is Iraq's second for a full four-year parliament since the 2003 U.S.-led invasion.

"We have recovered the pride, unity, sovereignty and security of Iraq. Now we just need to complete that task," Prime Minister Nuri al-Maliki told tribal leaders in Baghdad.

The Pentagon said on Wednesday only an "extraordinarily dire" security deterioration would warrant a slowdown in plans for the remaining 96,000 U.S. troops to end combat operations in August and withdraw completely by the end of next year.

White House spokesman Robert Gibbs said on Thursday the United States remained "strongly on track" to withdraw all U.S. combat brigades by the end of August.

Thursday's attacks occurred despite tight security measures imposed to guard the 950,000 people eligible to vote early, most of them from Iraq's 670,000-strong security for

ces.

Foreign oil firms starting to invest in Iraq's vast energy resources are watching to see if security gains can be sustained against Shi'ite militias, which the U.S. military says are backed by Iran, and Sunni Islamist militants like al Qaeda.

The al Qaeda-affiliated Islamic State of Iraq group threatened last month to prevent Sunday's election at any cost, using primarily "military" means to stop what it called a farce aimed at cementing Shi'ite domination over Sunnis.

EMPTY PROMISES

Security -- and broken pledges -- were high among issues listed by policemen voting in the Shi'ite holy city of Najaf.

"What happened to all the promises that were made?" said Raad Jaafar. "We don't want a leader who just gives guns to rich tribal sheikhs. Is this how Iraq wants to help the poor?"

He said he would not vote for Maliki, who did well in provincial elections in January 2009, when he put security improvements at the centre of his campaign.

But out of 15 policemen interviewed at a Najaf polling centre, most said they backed Maliki's State of Law coalition.

Campaigning for a second term, Maliki has claimed credit for lower levels of violence as sectarian bloodletting between once-dominant Sunnis and newly empowered majority Shi'ites recedes.

But a series of devastating suicide bombings in Baghdad has undercut the Shi'ite prime minister's security credentials.

Maliki faces a stiff challenge from his erstwhile Shi'ite partners and from former Premier Iyad Allawi's secular, cross-sectarian alliance.

At least 33 people were killed on Wednesday in volatile Diyala province, northeast of Baghdad, when three suicide bombers attacked police stations and a hospital.

A 27-year-old policeman who gave his name only as Jaafar said as he voted in Nimrud, a town near the violent northern city of Mosul: "Hopefully the guys coming in will do a better job than the last government, when we just got explosions."

Ali Nusaief, among dozens of police waiting at a dusty polling station in the southern oil city of Basra, said the election was the "last chance" for voters to make a change.

In the northern city of Kirkuk, a female Iraqi soldier showed up to vote with three women colleagues.

"I'm very happy to vote today," she said. "I hope the winning list will provide Iraqis with services and jobs."

The oil-producing city is at the heart of rival claims by Arabs and Kurds, which many analysts see as the most dangerous threat to Iraq's stability after the election.

Foreign Minister Hoshiyar Zebari, who is a Kurd, accused Iraq's six neighbours of trying to influence the election.

"This is not just an Iraqi election, this is a regional election that Iraq's neighbours are watching very closely," he told Reuters late on Wednesday.

(Additional reporting by Waleed Ibrahim in Baghdad, Fadhel al-Badrani in Falluja, Mohammed Abbas an

d Khaled Farhan in Najaf, Sherko Raouf in Sulaimaniya, Mustafa Mahmoud in Kirkuk, Ayla Jean Yackley in Arbil, Jack Kimball in Mosul, Aref Mohammed in Basra and Ross Colvin in Washington; Writing by Alistair Lyon; Editing by Andrew Roche and Peter Cooney)