BAGHDAD, Iraq Feb 7, 2007 (AP)— A Sea Knight helicopter crashed Wednesday in an insurgent stronghold northwest of Baghdad, killing all seven people on board, the military said, the fifth chopper lost in Iraq in just over two weeks.

A senior U.S. defense official said the 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-Qaida-linked Sunni group claimed responsibility for the downing.

The twin-rotor CH-46 went down about 20 miles northwest of the capital, said U.S. military spokesman Maj. Gen. William Caldwell. "A quick reaction force is on site and the investigation is going on as we speak," he told reporters in Baghdad.

The military said later that the Marine CH-46 helicopter went down in the volatile Anbar province while conducting routine operations and all seven crew members and passengers were killed in the crash.

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.

Caldwell also said the long-awaited Baghdad security operation "is ongoing as we speak," a day after Prime Minister Nouri al-Maliki acknowledged that 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.

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

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