BAGHDAD — The new U.S. commander in Iraq acknowledged Thursday that U.S.-led forces could not protect all Iraqis from "thugs with no soul" bent on reigniting sectarian warfare.

"Any student of history recognizes that there is no military solution to a problem like that in Iraq," Army Gen. David Petraeus said at his first news conference since assuming command last month.

Political negotiations were vital and would require reaching out to "some of those who have felt the new Iraq did not have a place for them," Petraeus said. "Military action is necessary to help improve security ... but it is not sufficient."

Petraeus' point was one U.S. officials have been making for several years as they pressed for Iraqis to assume control of their country after the U.S. overthrow of Saddam Hussein.

Previous attempts to unite Iraq's ethnic and religious communities behind the Shiite Muslim-led government have faltered, in part because of divisions over the role of private militias, allocation of resources and balance of power.

The U.S. and Iraqi governments have poured thousands of additional troops into Baghdad in the past month in a bid to clear out the Sunni Arab insurgents and Shiite militiamen driving the country's civil war.

Petraeus declined to specify how long the security operation would last but said it would continue as long as necessary "to achieve its desired effect."

"We are still in the early days of this endeavor — an endeavor that will take months, not weeks, to fully implement," Petraeus said.

Petraeus said it was too early to discern trends but noted some encouraging signs, including a drop in sectarian executions and the return of a small number of families to neighborhoods they had fled.

But he acknowledged that insurgents were stepping up attacks, including "barbaric" bombings and shootings that have killed more than 190 Shiite pilgrims as they headed to the holy city of Karbala for a weekend religious commemoration.

"Some sensational attacks inevitably will continue to take place, though every effort will be made to reduce their numbers," Petraeus said.

He said as many as 7 million pilgrims were believed to be on the roads, many of them on foot.

"It is an enormous task to protect all of them," Petraeus said, "and there is a point at which, if someone is willing to blow up himself, particularly perhaps disguise himself and use a vest rather than a vehicle, the problem becomes very, very difficult."

Police Thursday recovered the bodies of three elderly men, believed to be pilgrims, who were kidnapped and killed along the pilgrimage route through Sunni-dominated west Baghdad.

Some of the worst attacks of recent weeks have taken place outside Baghdad, in areas that are not covered by the security plan.

While the focus of the crackdown would remain Baghdad, Petraeus said, some additional troops would be deployed to the volatile province of Diyala and other outlying areas.

On Wednesday, the Pentagon approved Petraeus' request for 2,200 military police to handle the anticipated increase in detainees, Defense Secretary Robert M. Gates said in Washington.

"Right now we do not see other requests looming out there," said Petraeus, who stressed that it was still early and noted that the promised 21,500 additional U.S. troops would not be fully deployed in Baghdad and Anbar province until June.