A spokesman described the dead as "coalition forces members", and said the incident happened at about 2:15 a.m.
An investigation was under way and the cause of the incident was unknown, Major Jose Lopez said.
There are about 150,000 US military personnel deployed in Iraq. They are due to withdraw from Iraq by 2011.
The US spokesman gave no further details, but an Iraqi police general from the northern province of Salahuddin said two helicopters collided near Kirkuk, 250km north of Baghdad.
The last fatal crash involving US military aircraft happened in September, 2008.
A CH-47 Chinook helicopter crashed about 96km west of the city of Basra, killing seven US soldiers, Russian Peacekeeper reported.