Insurgents killed five troops in separate attacks in the deadliest day for U.S. forces since the war began.
The CH-53 Sea Stallion was carrying personnel from the 1st Marine Division when it crashed about 1:20 a.m. near the town of Rutbah, about 220 miles west of Baghdad, while conducting security operations, the military said in a statement.
A search and rescue team reached the site and an investigation into what caused the crash is under way.
President Bush expressed his condolences for the deaths at a press conference Wednesday morning. "The story today is going to be very discouraging to the American people. I understand that. It is the long-term objective that is vital -- that is to spread freedom," he said.
When asked if insurgents would scare many Iraqis away from the polls on Sunday, the president said "a lot of Iraqis" are expected to participate in the elections. "Clearly, there are some who are intimidated," he said. "I urge people to vote. I urge people to defy these terrorists."
Militants carried out at least six car bombings and other attacks on schools to be used as polling stations, political party offices and Kurdish sites -- killing or wounding more than two dozen people -- just days before Sunday's Iraqi national elections, the Associated Press reported.
In the Anbar province, four U.S. Marines were killed in fighting, the military said, providing no further details.
WABC reporter Jim Dolan, who was embedded with the troops who were attacked, said the deaths came when insurgents struck a vehicle with a rocket-propelled grenade as it was traveling with a Marine convoy out of the town of Haditha, west of Baghdad, according to the AP.
Insurgents also attacked a U.S. Army patrol near the northern town of Duluiyah on Wednesday, killing one soldier and wounding two others, the U.S. command said.
According to an AP count, the latest deaths -- if determined to all be military personnel -- would bring the total number of U.S. military who died in Iraq to 1,408.
Other attacks targeting political groups and voting sites included a suicide car bomb at the offices of the Kurdistan Democratic Party in the town of Sinjar, near Mosul, killing or wounding at least 20 people, said KDP official Mahdi al-Harki.
Gunmen opened fire 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 AP reported.
The KDP and PUK are the two largest Kurdish groups in Iraq and have formed a coalition with other Kurdish groups to run in the election.
Insurgents also set off three apparently coordinated car bombs in the town of Riyadh, north of Baghdad, killing at least five people, including three policemen.