BAGHDAD, Iraq (AP) - A U.S. helicopter crashed early Wednesday in a desert sandstorm, killing the 30 Marines and one sailor aboard, and six other troops were killed by insurgent ambushes in the heaviest single-day loss of life for the United States since the Iraq war began.
Four days before the country's crucial elections, militants set off at least eight car bombings that killed 13 people and injured another 40, including 11 Americans. The guerrillas also carried out a string of attacks nationwide against schools that will serve as polling centers.
In Washington, President George W. Bush asked Americans to remain committed to the Iraqi effort, despite what he called a ``very discouraging'' day when the U.S. death toll for the war rose above 1,400.
``We value life and we weep and mourn when soldiers lose their life,'' the president said. ``But it is the long-term objective that is vital, and that is to spread freedom.''
While a group calling itself al-Qaida in Iraq warned people to stay away from the polls - saying they would only have themselves to blame if they are hurt in attacks - Bush called on Iraqis to ``defy the terrorists'' and cast ballots in the Sunday election.
A Bush administration official said the cause of Wednesday's crash was not immediately known but that there was bad weather at the time. An Accuweather map of Iraq showed sandstorms Wednesday in the western region of Iraq, near the Jordanian border where the crash took place.
The CH-53 Sea Stallion, with a crew from the 3rd Marine Aircraft Wing, was ferrying personnel from the 1st Marine Division when it went down about 1:20 a.m. near the town of Rutbah, about 355 kilometers (220 miles) west of Baghdad in Anbar province, while conducting security operations, the military said in a statement.
A search and rescue team was sent to the site, and an investigation of the cause was underway.
The victims were 30 Marines and one sailor, said Lt. Gen. John Sattler, the top Marine commander in Iraq - the most American service members to die in a single incident in Iraq. Overall, it was the deadliest day for U.S. forces since the March 2003 invasion almost two years ago.

Four other U.S. Marines were killed in fighting in Anbar province, the military said in a statement.

A reporter embedded with those troops, Jim Dolan of WABC, said the deaths came when insurgents ambushed a Marine convoy leaving the town of Haditha, northwest of Baghdad, hitting a vehicle with a rocket-propelled grenade.

Also Wednesday, insurgents attacked a U.S. Army patrol in Duluiyah, 50 miles north of Baghdad, killing one soldier and wounding two others, the U.S. command said. Another U.S. soldier died and two others were wounded in a roadside bombing in the capital, the U.S. command said.
With the helicopter crash and other deaths, at least 1,409 U.S. military members have died in Iraq, according to an Associated Press count.
The previous single deadliest incident for U.S. troops was also a helicopter crash: a November 2004 collision of two Black Hawk helicopters that killed 17.
Previously, the most Americans killed in one day came on the invasion's third day - March 23, 2003 - when 28 troops were killed during the U.S. military's drive to take Baghdad and topple Saddam Hussein.
Last month, a suicide bomb exploded at a mess tent in a base near Mosul, killing 22 people including 14 U.S. soldiers and three American contractors.
The U.S. military has lost at least 33 helicopters since the start of the war, including at least 20 brought down by hostile fire, according to a study by the Brookings Institute.
Despite the violence, all sides pledged that the national elections set for Sunday would proceed.
Sunni Muslim extremists have threatened to sabotage the election and many Sunni clerics have called for a boycott because of the presence of U.S. and other foreign troops.
A string of political violence continued. Several schools slated to be used as polling stations were bombed overnight.
A suicide bomber detonated a fuel tanker at the offices of the Kurdistan Democratic Party in the town of Sinjar, southwest of Mosul, killing five and injuring at least 20 people, KDP officials said.
Earlier in the day, gunmen opened fire with machine guns on the local headquarters of the Patriotic Union of Kurdistan and the Communist Party in the city of Baqouba, north of Baghdad, killing a traffic policeman. The KDP and PUK are the two largest Kurdish groups in Iraq and have formed a coalition along with other Kurdish groups to run in the election.
Insurgents also set off three car bombs in rapid succession in the town of Riyadh, north of Baghdad, killing at least five people - including three policemen.
Four American soldiers were injured in a car bombing Wednesday in Saddam's hometown of Tikrit, the U.S. command said. Another car bomb targeted a multinational forces convoy on the road to Baghdad's international airport, injuring four soldiers, the command said.
The attack temporarily closed the airport road, one of the country's most dangerous.
Another car bombing later hit the same airport road, and an eighth car bomb detonated prematurely in the town of Mashahda, 30 miles north of Baghdad, killing the two men in the car.
A Web site statement, purportedly from al-Qaida in Iraq, said it carried out one of the attacks on the airport road, claiming that the targets were Americans.
The group also warned Iraqis to stay away from the polls Sunday.
``Oh people, be careful. Be careful not to be near the centers of infidelity and vice, the polling centers ... Don't blame us but blame yourselves'' if harmed.
The statement's authenticity could not be verified.