Seventeen coalition soldiers have been killed and five wounded in a mid-air collision between two US helicopters.
There have been a series of US helicopter losses in recent weeks
The Black Hawk helicopters - both from the 101st Airborne Division - crashed over the northern city of Mosul.
Witnesses say one of the helicopters was hit by a missile and spun out of control into the other, but officials have not commented on the cause.
The crash came as the US-appointed Governing Council unveiled an accelerated timetable to Iraqi control.
US Defence Secretary Donald Rumsfeld insisted that the new schedule for transferring power to Iraqi hands would not affect the military presence.
"This has nothing to do with coalition troops in Iraq," he told reporters in Japan where he is on a visit.
The crash comes two weeks after an American Chinook helicopter was shot down near Baghdad killing 16.
The BBC's Peter Greste in Baghdad says that if this latest disaster proves to be the result of an attack, it will once more underline just how wide the insurgency is spreading.
Well organised insurgents
The aircraft crashed over a residential part of Mosul shortly after dark, the American military said in a statement.
One soldier is still unaccounted for.
"The cause of the incident is under investigation," the military added.
However some eyewitnesses said a rocket-propelled grenade or missile had struck one of the helicopters.
A man quoted by Reuters news agency said he had seen the two helicopters collide after an explosion.
"I looked outside the window and saw two helicopters," he said.
"One was flying low and was on fire. The other was higher up. The first one climbed and hit the higher one."
Mosul is located well to the north of the so-called "Sunni triangle" that has been at the heart of the insurgency.
However the city is home to both Sunni Muslims and Kurds, and in recent weeks coalition forces in the region have been fighting an increasingly well organised resistance.
On Saturday the Iraqi Governing Council said the US-led coalition would hand over power to a transitional government by June 2004.
The announcement came after Iraqi leaders met US chief administrator Paul Bremer in Baghdad.
Bremer (centre) plans to end US occupation next year
Mr Bremer had earlier returned from the US, where plans for a faster handover were agreed at the White House.
The transitional body will prepare for a full sovereign Iraqi government by 2005, following a general election.
The plan is a much faster route to Iraqi sovereignty than the one previously laid out.
Council President Jalal Talabani - speaking after the meeting with Mr Bremer - said the transitional body would be selected after consultations with "all parties" in Iraqi society.
Sunni Muslim council member Adnan Pachachi said the move would "restore sovereignty" and give "a chance to a representative of the Iraqi people to represent Iraq".
President Bush described the revised handover plan as "an important step toward realising the vision of Iraq".