From combined dispatches
BAGHDAD -- A U.S.-Iraqi offensive against militants in Baghdad will begin within days and take place on a scale never seen during four years of war, American officers said yesterday.
A military spokesman, meanwhile, acknowledged for the first time that hostile fire was probably to blame for the crashes of four helicopters in the past two weeks, and said the U.S. command has ordered changes in flight operations.
Briefing a small group of foreign reporters, three American colonels who are senior advisers to the Iraqi army and police in Baghdad said a command center overseeing the crackdown in the capital would be activated today.
"The expectation is the plan will be implemented very soon thereafter," a senior adviser to the 9th Iraqi army division said at a U.S. military base in Baghdad.
"It's going to be an operation unlike anything this city has seen," Col. Doug Heckman added. "It's a multiple-order magnitude of difference, not just a 30 percent, I mean a couple hundred percent" larger than previous offensives.
The job of stabilizing Baghdad had become increasingly urgent after a wave of violence that, an Interior Ministry official said, claimed nearly 1,000 lives in the past week alone -- about135 of them in a single truck bombing on Saturday. At least 103 persons were killed or found dead yesterday, most of them in Baghdad, police reported.
The officers said the new security operation will be headed by Lt. Gen. Abboud Gambar, a Shi'ite in his early 60s who was named to the post under pressure from the U.S. military. Prime Minister Nouri al-Maliki initially had selected Lt. Gen. Mohan al-Freiji for the job.
The plan will involve U.S. and Iraqi forces sweeping the capital's neighborhoods for militants and illegal weapons and then holding cleared areas. But some analysts fear that as in previous crackdowns, militants will simply melt away and wait out the forces or strike in areas where they are not deployed.
The four helicopters that have gone down since Jan. 20 crashed after claims by insurgents that they have received new stocks of anti-aircraft weapons -- and a recent boast by Sunni militants that "God has granted new ways" to threaten U.S. aircraft.
Maj. Gen. William Caldwell, the chief military spokesman in Iraq, told reporters that the investigations into the crashes of three Army helicopters and one private copter were incomplete, but "it does appear they were all the result of some kind of anti-Iraqi ground fire."
Despite the losses, Gen. Caldwell said it was premature to conclude that the threat to U.S. aircraft posed by Sunni insurgents and Shi'ite militiamen had increased dramatically.
"There has been an ongoing effort since we've been here to target our helicopters," Gen. Caldwell said. "Based on what we have seen, we're already making adjustments in our tactics and techniques and procedures as to how we employ our helicopters."
Gen. Caldwell did not elaborate, presumably for security reasons. In the past, defensive measures have included flying lower and faster, varying routes and using zigzag patterns over dangerous areas.
Three crashed in mostly Sunni areas and the fourth was shot down during fighting with members of a Shi'ite messianic cult near Najaf. U.S. officials have accused Iran of providing sophisticated weapons to Shi'ite militants.
In December, a spokesman for the late dictator Saddam Hussein's ousted Ba'ath Party, Khudair al-Murshidi, said in Damascus, Syria, that Sunni insurgents had received shoulder-fired anti-aircraft missiles, and "we are going to surprise them," meaning U.S. forces.
Mr. al-Murshidi did not say when or how the missiles were obtained.
Insurgents have used SA-7s, a shoulder-fired missile with an infrared homing device, against U.S. and British aircraft since 2003.
U.S. military helicopters are equipped with long-range sensors and devices to jam radar and infrared technology, but they have proven vulnerable to intense gunfire, as well as rocket-propelled grenades.