BAGHDAD, Iraq - A Sea Knight helicopter went down northwest of Baghdad on Wednesday, the military said, the fifth helicopter lost in Iraq in just over two weeks. Meanwhile, a US military spokesman said the Baghdad security operation is in progress.

US Army Black Hawk helicopter flies over Baghdad's heavily fortified Green Zone, Iraq, Wednesday, Feb. 7, 2007. The US military said it was investigating reports that an aircraft went down Wednesday and the reports came five days after a US Army helicopter crashed in a hail of gunfire north of Baghdad, - the fourth helicopter lost in Iraq in a two-week span. [AP]

The CH-46 helicopter went down about 20 miles northwest of the capital, US 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. "It would probably be inappropriate for me to talk about whether or not there are or are not casualties."

Witnesses said the helicopter had been shot down in a field in the Sheik Amir area northwest of Baghdad, sending smoke rising from the scene, in a Sunni-dominated area between the Taji air base, 12 miles north of Baghdad, and Garma, 20 miles west of the capital. and Garma, 20 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."

The reports came five days after a US Army helicopter crashed in a hail of gunfire north of Baghdad, police and witnesses said. The US command said two crew members were killed in that crash, and the al-Qaida-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 spate of helicopter crashes underscores the dangers facing US troops as they step up their presence in the Baghdad area in preparation for a long-awaited security sweep to quell the spiraling sectarian violence.

More American troops were killed in combat in Iraq over 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 US soldiers and Marines find themselves fighting more battles in the streets of Baghdad, as well as other cities.

The Iraqi government also has faced increased pressure over delays in starting the operation as hundreds of Iraqis have been killed in a spate of bombings and other violence in recent weeks.

At least 15 people were killed in attacks nationwide on 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.

Prime Minister Nouri al-Maliki acknowledged Tuesday that the long-awaited Baghdad security operation was off to a slow start and warned that insurgents were taking advantage of the delay to kill as many people as possible. But he also reassured Iraqis that security forces will live up to their responsibilities.

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 US and Iraqi forces were gearing up for a major neighborhood-to-neighborhood sweep to quell sectarian violence in the city of 6 million.

"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 in a meeting with military commanders shown 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."

Al-Maliki urged his commanders to step up efforts to complete the preparations for the security plan, saying the delays had allowed insurgents to step up attacks that have killed hundreds in recent weeks.

Defense Secretary Robert Gates said the increase in US forces in Iraq is "not the last chance" to succeed and conceded he was considering what steps to take if the buildup fails.

"I would be irresponsible if I weren't thinking about what the alternatives might be," he told the Senate Armed Services Committee.

Gates said the operation was to have started Monday. "It's probably going to slip a few days, and it's probably going to be a rolling implementation," he said.

Al-Maliki, who has seen sectarian violence rise since taking office on May 20, 2006, despite two previous efforts to secure the capital, declared that Iraqi forces will live up to their responsibilities and told his commanders they must not disappoint those "who stand beside us."

"As far as the security issue is concerned, we should be determined and committed. We should carry out the operation on time and should not delay because the delay will be used against us by our enemies," he added.

Al-Maliki also accused other countries in the region of supporting militants to destabilize Iraq and prevent democracy from spreading - an apparent reference to US rivals Iran and Syria.

"We have many times talked about this interference and said that we will not sit endlessly silent about those who interfere in our affairs and support terrorism," he said.