Two US soldiers were killed when their Kiowa observation helicopter crashed on Friday evening in southwest Baghdad, the US military said on Saturday.
"The cause of the crash has yet to be determined," a military statement said.
Five US soldiers and six Iraqis were also killed in a string of attacks on Friday, as fighters continued their bloody campaign despite the government's claim it had netted three aides to Abu Musab al-Zarqawi, Al-Qaida's purported frontman in Iraq.
Meanwhile, attackers fired mortars at an Iraqi National Guard barracks in the central city of Suwayra, killing one Iraqi soldier and wounding another, the Polish military said on Saturday.
US tanks and troops patrol Iraqi
streets before the elections
The attack occurred at 10:30pm (1930 GMT) on Friday when eight mortar rounds exploded at the barracks, the Polish-led force in Iraq said in a statement.
The National Guard members responded with mortars of their own, fired in the direction of the attackers. The attack then stopped, the statement said.
Poland leads an international force of 6000 troops in central Iraq whose duties involve patrolling and training Iraqi security forces.
Polling station attacked
A bomb exploded on Saturday close to a US and Iraqi security centre near a polling station in the Iraqi town of Khanaqin, northeast of Baghdad, an election commission official in the province said.
Local police said at least five people had been killed.
Abd al-Jalil Adil initially said the polling station had been targeted, but later modified this to say the target might have been the security centre.
US forces sealed off the site.
Over the past week, dozens of polling stations in Iraq have been attacked by various armed groups.
The Iraqi authorities shut down Baghdad international airport and closed its land borders for three days, in a bid to control the country for Sunday's general elections, the first since Saddam Hussein was toppled in April 2003.
Iraqi security forces are using
new armoured vehicles
A dusk-to-dawn curfew was also slapped on most of the country and vehicle restrictions were enforced, as security and police urged residents in Iraq's danger spots to stay at home until election day.
Since Wednesday, at least 54 Iraqis and 15 US troops have been killed in insurgent attacks. A helicopter crash also killed 30 US Marines and one sailor on Wednesday, the deadliest single incident of the war for the US military.