The U.S. military relies heavily on helicopters to ferry troops, dignitaries and supplies to avoid the threat of ambushes and roadside bombs, and Thursday's crash highlighted the noncombat dangers facing Americans in Iraq.
In all, 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.
Maj. John Hall, a military spokesman in Baghdad, said hostile fire had been ruled out in Thursday's crash and that the three other helicopters suffered no damage.
A Pentagon official in Washington said it appeared that the twin-engine transport aircraft had malfunctioned.
"They think it was a mechanical problem," the official said on condition of anonymity for lack of authorization to speak to the press on the record.
The Chinook, considered the Army's workhouse, has the capacity to carry more than 30 people, but the military said the seven killed were the only ones on board the helicopter that crashed.
The aircraft - which was en route to a base in Balad, north of Baghdad - went down in an area under British military control. A British quick reaction force and road convoy were dispatched to help American officials at the site, officials said.
It was the deadliest helicopter accident for U.S. troops since Aug. 22, 2007, when a UH-60 Black Hawk helicopter crashed in northern Iraq, killing all 14 U.S. soldiers aboard.
"It is a tough day for the coalition and we are deeply saddened by the loss of our soldiers," said Col. Bill Buckner, another military spokesman. "Our prayers and condolences go out to the families during this difficult and tragic incident."
The military did not release the names or hometowns of those killed pending notification of next of kin. But Alex Weintz, spokesman for U.S. Rep. Mary Fallin, R-Okla., said all seven were National Guardsmen - four from Texas and three from Oklahoma.
The military also reported the deaths of two soldiers in separate incidents Wednesday, neither related to combat.
At least 4,168 members of the U.S. military have died in Iraq since the war started, according to an Associated Press count.
Also Thursday, a roadside bomb killed three Iraqi soldiers in the northern city of Mosul, a police officer said on condition of anonymity because he was not authorized to speak to the media.
Prime Minister Nouri al-Maliki, meanwhile, stepped up pressure for U.S. officials to respond to Iraqi proposals for a security agreement.
Any pact must be ratified by the Iraqi parliament by Dec. 31, when the U.N. mandate for foreign forces expires, and prolonged deliberations are raising concerns the year-end deadline won't be met.
The prime minister also said the U.N. mandate would be extended only on Iraq's terms, raising a possible alternative to an agreement.
"The situation on the ground indicates that we both are in a critical situation," al-Maliki said Wednesday in remarks broadcast on Iraqi state TV.
Al-Maliki said the Americans had asked for two weeks to study Iraqi demands and that the time was up.
"Up to now, the U.S. team has not returned with an answer," the Iraqi leader said. "There are serious sticking points."
He insisted the Americans had agreed to withdraw all foreign troops by the end of 2011 but reiterated that the main obstacles were U.S. insistence that American soldiers should have immunity from Iraqi courts and Iraqi demands to gain control over U.S. raids and detentions.
In Washington, a senior U.S. official familiar with the negotiations, agreed that obstacles remained to an agreement.
The official, who declined to be identified in discussing the sensitive negotiations, stressed that no final agreement could be reached on a timeline for U.S. withdrawal until the issues of immunity and oversight are resolved.
