A U.S. military spokesman in Baghdad says the city's new security crackdown has started and the operation will continue to grow as more forces arrive.  VOA's Barry Newhouse reports from northern Iraq that military officials also announced that another U.S. helicopter has crashed - the fifth in two weeks.

Maj. Gen. William Caldwell gestures during press conference in heavily-fortified Green Zone in Baghdad, Iraq, 7 Feb 2007

U.S. military spokesman Major General William Caldwell told reporters in Baghdad the operation to end the city's worsening sectarian violence is underway.

"The implementation of the prime minister's plan has already begun and will be fully implemented at a later date," he said.

The plan calls for units of American and Iraqi forces to patrol Baghdad's most violent neighborhoods and to drive out the city's powerful sectarian militias.  The operation is the key part of President Bush's so-called "troop surge" aimed at reversing Iraq's slide into sectarian war.

General Caldwell says the Baghdad operation will broaden when more troops are available.

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

Nouri al-Maliki (file photo)

On Tuesday, following complaints from Baghdad residents about the city's continuing violence, Iraqi Prime Minister Nouri al Maliki admitted the Baghdad security push is starting late and urged Iraqi forces to speed up the operation.

A day later, a Pentagon spokesman in Washington cautioned against looking for a specific start date for the security plan.  Bryan Whitman said the operation had started "some time ago" and the troop buildup would gradually continue until May.

U.S. General David Petraeus, the author of the Army's new counter-insurgency warfare manual and a public supporter of the troop buildup, officially takes control of U.S. forces in Iraq on Saturday.

Crash site of a US military CH-46 Sea Knight helicopter which went down about 30 kilometers northwest of Baghdad, 7 Feb 2007

Meanwhile, military officials said another American helicopter crashed 30 kilometers northwest of Baghdad.  Officials offered no details on what caused the CH-86 transport helicopter to crash.  An al-Qaida-linked Sunni group claimed responsibility for the downing.

In the past two weeks, insurgents have shot down four other U.S. helicopters, killing at least 21 people.