Thirty US marines and a sailor have died in a helicopter crash in western Iraq, in the deadliest day for US troops since the March 2003 invasion.
Six US troops and several Iraqis were killed elsewhere in Iraq.
The helicopter came down in bad weather near the Jordanian border. A leading US general said there was no indication of hostile action.
It comes four days before Iraq's election - a poll that US President George Bush called a "grand moment".
"I urge all people to vote. I urge people to defy these terrorists," Mr Bush told reporters in Washington.
He expressed sorrow at the helicopter crash but said the details of what happened were still being investigated by defence officials.
US HELICOPTER LOSSES IN IRAQ
15 Nov 03: Two Black Hawks collide while trying to avoid ground fire in Mosul, killing 17
2 Nov 03: Chinook shot down near Falluja, killing 16
8 Jan 04: Black Hawk shot down near Falluja, killing nine
9 Nov 03: Black Hawk comes down near Tikrit, killing six
"Anytime we lose lives, it is a sad moment," he said.
In separate incidents, four US marines were killed in combat in restive Anbar province - the same province where the helicopter crashed - and a US soldier was killed when an army patrol came under attack in northern Iraq.
In Baghdad, another soldier was killed and two injured in a roadside bombing, taking the day's US death toll to 37.
The helicopter, a CH-53E Super Stallion, was flying over the desert when it went down at about 0120 local time (2220 GMT Tuesday), near the town of Rutbah.
Gen John Abizaid, commander of US troops in Iraq, said the helicopter was on "a routine mission in support of the elections".
There have been a series of attacks across the country as tensions increase ahead of Sunday's vote:
- At least 15 people are killed in car bomb explosion outside Kurdistan Democratic Party offices just outside the northern city of Mosul
- At least five people are killed by three car bombs in the town of Riyadh, near the northern city of Kirkuk
- Insurgents attack party political offices in Baquba, north of Baghdad - at least one police officer is killed
- A video recording released by insurgents in Mosul appears to show three captive Iraqis, who say they are election workers
- One person is killed and two wounded in car bomb explosion outside passport offices in Tikrit, police tell AFP news agency
- Four US soldiers are injured in a car bombing in Tikrit, while four more are hurt in a car bomb on the road to Baghdad's airport
This week, a senior US general said the army expected to keep about 120,000 troops in Iraq for at least two more years.
CH-53E SUPER STALLION
Largest US military helicopter, introduced in 1981
Can be used in all weathers
US Marines have a fleet of 160
Operated by a crew of three
Normally carries 37 troops - can be configured to carry 55
Troop numbers were recently boosted to 150,000, to improve security during the Iraqi election.
This latest helicopter crash is the worst single loss of life for the US military since the war began.
Until this latest incident, the worst helicopter crash was on 15 November 2003, when two Black Hawks collided under fire over Mosul, killing 17 service personnel.
The previous deadliest day for the US was 23 March 2003 when 28 troops were killed in combat.