The US army in Iraq is examining the wreckage of two Black Hawk helicopters which crashed in the northern city of Mosul, killing at least 17 soldiers.
The Black Hawks came down in a residential area
It was the worst US helicopter crash in Iraq so far - but officials are not yet able to say precisely what caused it.
Some witnesses said one of the helicopters was hit by ground fire and then collided with the second aircraft.
Five troops were injured and one is still missing after the crash, which hit 101st Airborne Division forces.
Five US helicopters have crashed in the past three-and-a-half weeks in Iraq with the loss of 39 lives.
In Mosul, the Black Hawks crashed within 250 metres of each other.
The US-led coalition has not yet confirmed the nationalities of those on board.
It said one helicopter was carrying a quick reaction force and the other was carrying soldiers on a transport mission.
According to an unconfirmed report, the tail of one helicopter was hit by a rocket-propelled grenade.
Reports are coming in that US forces have fired a satellite-guided missile at a "former regime training area" in northern Iraq, for the first time since the end of major combat.
Lieutenant Colonel Bill MacDonald told AFP the firing of the missile marked the launch of a crackdown in north-central Iraq, Operation Ivy Cyclone II.
US HELICOPTER CRASHES IN IRAQ
15 Nov - Two Black Hawks crash in Mosul, at least 17 killed
7 Nov - Black Hawk shot down near Tikrit, six killed
2 Nov - Chinook shot down near Falluja, killing 16 US soldiers
25 Oct - Black Hawk crashes near Tikrit, reportedly hit by ground fire - soldier injured
The helicopter crashes came as the US-appointed Governing Council unveiled an accelerated timetable to transfer the country to Iraqi control.
US Defence Secretary Donald Rumsfeld insisted that the new schedule would not affect the US military presence.
"This has nothing to do with coalition troops in Iraq," he told reporters in Japan where he is on a visit.
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.
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.
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".