The Marine Corps transport helicopter came down in the western desert near the Jordanian border with the loss of all 31 marines on board, a marine official said.
"Three are no indications as to the cause of the crash," the official said, adding that hostile fire had yet to be ruled out as a cause.
General John Abizaid, the commander of US forces in the region, told reporters there were no survivors. "Weather was bad. We don't know of any enemy action. The investigation will have to continue. There will have to be more that comes out of this to learn what happened."
Abizaid said the marines were on "a routine mission in support of the elections."
Four other marines were reported killed in action on Wednesday in western Al-Anbar province and a soldier was killed in an attack on a patrol north Baghdad, raising the US death toll for the day to 36, the highest single-day US toll of the war.
"The story today is going to be very discouraging to the American people," Bush told reporters in Washington at his first news conference of his second term of office.
"I understand that. We value life. We weep and mourn when soldiers lose their life."
Bush promised that US troops would stay in Iraq no longer than needed to stabilise the country but stopped short of announcing any timetable for withdrawal.
"We will complete the mission as quickly as possible," he said. "Obviously we're going to have a troop level necessary to complete the mission, and that mission is to enable Iraq to defend itself from terrorists."
Bush urged Iraq's estimated 14.2 million eligible voters to brave insurgent death threats on Sunday and take part in the first post-Saddam Hussein elections.
"We anticipate a lot of Iraqis will vote... I anticipate a grand moment in Iraq history," he said.
Echoing Bush, Iraq's top military commander, joint chief of staff General Babakir Zebari, said Iraqi forces would take full security responsibility by December, with US troops out of the cities and scaled back to possibly one or two bases nation-wide.
A senior US defence official confirmed that Washington envisaged re-deploying its troops away from population centres by year-end but declined to say how many bases would be retained.
A woman and a child were among 15 people killed when a truck bomb exploded outside the offices of one of the main Kurdish parties in the north-western town of Sinjar, a municipal official said.
The official said a tonne of explosives were used in the attack against the offices of the Kurdistan Democratic Party.
Miliants loyal to al Qaeda Iraq frontman Abu Musab al-Zarqawi said they carried out the bombing.
In a string of messages posted on the Internet in recent days, Zarqawi's organisation has vowed to sabotage Sunday's election and has hit out at Iraq's long oppressed Shia majority, which is expected to sweep the polls.
"Beware, beware, Iraqis, don't approach the (polling) centres of infidelity and vice. You are warned," said one statement.
In the town of Samarra, north of Baghdad, three Iraqis, including a woman and a child, were killed in a bomb blast, police said.
In the so-called triangle of death just south of the capital, two Iraqis were killed in a US operation in the town of Mahmudiyah. Two Shias were also shot dead in the town in circumstances that were not immediately clear.
As polling day neared, insurgents stepped up their attacks on schools and other buildings designated as polling stations, attacking 13 in the space of 24 hours.
Amid the violence, Interior Minister Falah Naqib hailed the arrest of a suspected henchman of Zarqawi pledged reward money of up to 200,000 dollars to security force personnel who net wanted militants.
The man, known only as Abu Saada, "is a Zarqawi assistant. He took part in many crimes in Salaheddin and Nineveh (provinces)... He killed many people," Naqib said.
"We are now beginning to dismantle the terrorist cells," he said, adding that nine other suspects were also arrested.