The military said the crash "does not appear to be by enemy action."
No precise location was given for the 2:15 a.m. crash, but a military spokesman said it occurred in Tamim province, which includes the oil-rich disputed city of Kirkuk.
Iraqi officials said the crash site was located about 20 miles (30 kilometers) west of Kirkuk, which is about 180 miles (290 kilometers) north of Baghdad. They spoke on condition of anonymity because they were not supposed to release the information.
Maj. Derrick Cheng, a spokesman for U.S. forces in northern Iraq, said all the dead were Americans. He declined to give more details.
The deaths raised to at least 4,236 the number of U.S. service members who have died in the Iraq war since it began in March 2003, according to an Associated Press count.
The number of Americans killed in Iraq has dropped significantly with an overall decline in violence in the country.
Monday's crash was the deadliest single incident for U.S. troops since Sept. 18, when seven American soldiers were killed in a helicopter crash in the southern desert west of Basra.
The U.S. military relies heavily on helicopters and other aircraft to ferry troops, dignitaries and supplies to avoid the threat of ambushes and roadside bombs in Iraq.
At least 70 U.S. helicopters have gone down since the war started in March 2003, according to military figures. Of those, 36 were confirmed to have been shot down.
The most recent previous incident was on Nov. 15, when a helicopter made a hard landing after hitting wires in the northern city of Mosul, killing two American soldiers.
A Russian-made cargo plane chartered by FedEx also crashed in November after reporting a malfunction west of Baghdad, leaving the seven crew members dead.
The Jan. 2005 crash of a U.S. Marine CH-53 Sea Stallion helicopter in western Iraq claimed 31 lives - the biggest single U.S. loss of life in the Iraq war. Investigators determined the crash was not due to hostile fire.
Iraqi electoral officials, meanwhile, geared up for Saturday's provincial elections - the first nationwide vote in more than three years.
A spokesman for Iraq's Independent High Electoral Commission, Qassim al-Aboudi, said the panel had punished more than 69 parties or coalitions for 180 campaign violations ranging from putting posters outside allocated locations and defaming rivals.
He expressed concern that was only a preview for likely conflicts and claims of fraud after the vote.
"I think that this issue will get worse after the results are declared," he said at a news conference. "I do not expect that the losers in the elections will congratulate the winners."
"They will not miss any chance to question the integrity of the results. But practically speaking, we have taken all necessary measures to combat fraud," he said.
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Associated Press writer Sameer N. Yacoub contributed to this report.
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