BAGHDAD, Iraq — A Sea Knight helicopter went down northwest of Baghdad today, the military said, the fifth helicopter lost in Iraq in just over two weeks.

The CH-46 helicopter went down about 20 miles northwest of the capital, U.S. military spokesman Maj. Gen. William Caldwell said, but he declined to comment on casualties.

"A quick reaction force is on site and the investigation is going on as we speak," he said in Baghdad, without giving a cause for the crash. "It would probably be inappropriate for me to talk about whether or not there are or are not casualties."

Caldwell also said the long-awaited Baghdad security operation "is ongoing as we speak," a day after Iraqi 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.

The Iraqi officer who is leading the security drive, Lt. Gen. Abboud Gambar, took over the operation headquarters on Monday, but there had been no announcement until today that the sweep, the third attempt to crush violence in nine months, had begun.

"The implementation of the prime minister's plan has already begun and will be fully implemented later, having all the parts and pieces that he wants," Caldwell said. "But 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."

President 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 in the Baghdad area.