A Sea Knight helicopter crashed Wednesday northwest of Baghdad, killing all seven people on board, the military said, the fifth chopper lost in Iraq in just over two weeks.

The military said the Marine helicopter went down in Anbar province, an insurgent stronghold west of Baghdad, while conducting routine operations and all seven crew members and passengers were killed in the crash.

It did not give a cause for the crash. A senior U.S. defense official said the CH-46 helicopter did not appear to have been hit by hostile fire, but an Iraqi air force officer said it was downed by an anti-aircraft missile and an Al Qaeda-linked Sunni group claimed responsibility for the downing.

Meanwhile, the long-awaited Baghdad security operation has begun, said U.S. military spokesman Maj. Gen. William Caldwell, a day after Prime Minister Nouri al-Maliki acknowledged the plan to pacify the violence-ridden capital had been slow to start and had allowed insurgents time to step up attacks that have killed hundreds of Iraqis in recent weeks.

The U.S. official, who spoke on condition of anonymity because the investigation was still under way, said the crash appeared to have been related to mechanical problems and all seven people aboard had died.

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The twin-rotor helicopter was operated by Marines, and other Marine aircraft were in visual contact at the time it went down, the U.S. official said, but he did not know whether a distress signal was communicated by radio.

The helicopter went down about 20 miles northwest of the capital, Caldwell said, although he declined to comment on casualties or give a cause for the crash.

"A quick reaction force is on site and the investigation is going on as we speak," he told reporters in Baghdad.

U.S. forces sealed off the area and helicopters buzzed overhead as flames and a huge plume of black smoke billowed from the wreckage in an open field, not far from a squat concrete farmhouse.

U.S. military officials have said the Baghdad operation began to be put in place when President Bush announced it Jan. 10, and Caldwell said Wednesday it was "ongoing as we speak." Officials have said there would be no announced start of the security sweep but instead it would build gradually.

The Iraqi officer who is leading the security drive, Lt. Gen. Abboud Gambar, took over the operation headquarters Monday.

"Portions are already being put in place, and we'll continue to put more into place as the forces arrive and the assets become available," Caldwell said.

Bush is increasing the number of U.S. troops in Iraq by 21,500, including 17,500 for Baghdad, as part of the efforts. But the series of helicopter crashes underscores the dangers facing U.S. troops as they step up their presence.

The Iraqi air force officer, who was familiar with the helicopter investigation but spoke on condition of anonymity because he was disclosing confidential information, said the chopper went down after it was hit by an anti-aircraft missile.

Witnesses also said it had been shot down in a field in the Sheik Amir area northwest of Baghdad in a Sunni-dominated area between the Taji air base, 12 miles north of Baghdad, and Karmah, 50 miles to the west of the capital.

"The helicopter was flying and passed over us, then we heard the firing of a missile," said Mohammad al-Janabi, a farmer who was speaking less than a half-mile from the wreckage. "The helicopter, then, turned into a ball of fire. It flew in a circle twice, then it went down."

Iraqi insurgents have used heavy machine guns, rocket-propelled grenades and shouldered-fired SA-7 anti-aircraft missiles throughout the conflict. U.S. officials believe Iran is supplying Shiite militias with new weapons including more powerful roadside bombs, Katyusha rockets and a newer class of RPGs.

Some of those weapons could have found their way into the hands of Sunni insurgents, who operate around Taji.

The U.S. military relies heavily on helicopters to avoid roadside bombs and insurgent ambushes. Any new threat to helicopters would be a serious challenge.

The CH-46 is used by the Marines primarily as a cargo and troop transport, and can carry 25 combat-loaded troops, according to the think tank GlobalSecurity.org.

The claim of responsibility came in an Internet statement signed by the Islamic State in Iraq, an umbrella group of several Sunni insurgent groups, including al-Qaida in Iraq. The same group claimed responsibility for downing two other helicopters recently.

The authenticity of the statement — posted on a Web forum where the group often issues statements — could not be independently confirmed.

The Sea Knight went down five days after a U.S. Army helicopter crashed in a hail of gunfire north of Baghdad, police and witnesses said. The U.S. command said two crew members were killed in that crash, and the Al Qaeda-affiliated group the Islamic State of Iraq claimed responsibility.

Three other helicopters also have gone down since Jan. 20 killing a total of 19 Americans — 14 troops and five civilian security contractors.

The military has said all four were believed to have been shot down, raising new questions about whether Iraqi insurgents are using more sophisticated weapons or whether U.S. tactics need changing.

Gen. Peter Pace, chairman of the Joint Chiefs of Staff, has acknowledged that insurgent ground fire in Iraq has been increasingly effective.

"I do not know whether or not it is the law of averages that caught up with us or if there's been a change in tactics, techniques and procedures on the part of the enemy," he told a congressional hearing on another subject Tuesday, adding the downings were being investigated.

More American troops were killed in combat in Iraq in the past four months — at least 334 through Jan. 31 — than in any comparable stretch since the war began, according to an Associated Press analysis of casualty records, as U.S. soldiers and Marines find themselves fighting more battles in the streets of Baghdad, as well as other cities.

The military said a Marine was killed Tuesday in fighting in Anbar province, an insurgent stronghold west of Baghdad.

At least 15 Iraqis also were killed in attacks nationwide Wednesday, including two employees of the government-funded Iraqi Media Network in Baghdad and a female government official who was shot to death while she was riding to work with her husband in the northern city of Mosul.

Al-Maliki acknowledged Tuesday the Baghdad security operation was off to a slow start, but he reassured Iraqis that security forces will live up to their responsibilities, telling his commanders they must not disappoint those "who stand beside us."

"The operations will unite us and we will take action soon, God willing, even though I believe we've been very late and this delay has started to give a negative message," al-Maliki said on state TV. "I hope that more efforts will be exerted and more speed exerted in carrying out and achieving all the preparations to start the operations."

The statement came as new checkpoints were erected and increased vehicle inspections and foot patrols were reported in some neighborhoods — providing the main evidence so far that U.S. and Iraqi forces were gearing up for a major neighborhood-to-neighborhood sweep to quell sectarian violence in the city of 6 million.

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