The unrest came as the United States voted for its next president after an election campaign that has focused closely on Washington's decision to invade Iraq.

Upping the stakes, at least four staff from a foreign company, including an American, remained missing after kidnappers raided their Baghdad offices on Monday.

Prime Minister Iyad Allawi has pledged to restore order to the country ahead of Iraq's own landmark polls, promised by January, and is prepared to use force.

To this end, US and Iraqi troops are massing around the flashpoint cities of Fallujah and neighbouring Ramadi with expectations high of an all-out assault against militants who strike the government and its allies with a daily barrage of attacks.

At least six people were killed and another 21 wounded when a suicide car bomb tore through a line of cars outside the education ministry in northern Baghdad at about 9:30 am (0630 GMT), medics and police officials said.

"It was a suicide attack, we found the body of the driver inside the charred remains of the car," said police lieutenant Bassam Bahr.

The bomber attempted to ram the gates outside the heavily-protected ministry compound but was stopped by guards before detonating his charge, one of the guards on duty said.

Minutes earlier another car bomb exploded in western Baghdad on a US military patrol, wounding one soldier and damaging an Iraqi truck, the army said in a statement.

Two national guardsmen were also hurt when a roadside bomb exploded as their convoy drove through the town of Abu Ghraib on the western outskirts of the capital.

Adding to the bloodshed, a national guard commander, General Rashid Falih, and at least one other guardsman died and 11 others were wounded in two separate car bomb attacks in Iraq's main northern city of Mosul, Iraqi police officials said.

In a separate crisis, four people -- an American, a Filipino, a Nepalese and an Iraqi -- remained missing after kidnappers raided a Saudi trading company's Baghdad offices on Monday, another employee said.

Meanwhile, an Iraqi cameraman working for Reuters was shot dead during clashes in rebel-held Ramadi Monday, while a US war plane bombed nearby Fallujah overnight, the military and news agency said Tuesday.