The new US commander in Iraq, General David Petraeus, said on Thursday the reinforcements pouring into Baghdad will stay "well beyond the summer" and warned there is no military solution to conflict.
The general also said that Iraqi leaders would eventually have to sit down and talk with some of the violent factions tearing the country apart.
Braving the sectarian terror, thousands of Shiite pilgrims were streaming to Karbala to join about 2.5 million devotees already in the holy city for the sombre religious ceremony of Arbaeen on Friday.
At his first news conference since taking charge of US-led forces in Iraq, Petraeus said he felt "shame, horror and sorrow" when he heard of a suicide attack on Tuesday that killed at least 117 Shiite pilgrims .
He said "thugs with no souls" were carrying out atrocities to derail Operation Fardh al-Qanoon (Imposing Law), launched with 90,000 Iraqi and US troops last month in a bid to quell sectarian violence in Baghdad.
The US military is implementing a "surge" of more than 21,500 extra troops into Baghdad and neighbouring Anbar province in support of the plan.
But the general cautioned against expecting too much from the military and warned the security plan would take months to complete.
"There is no military solution to a problem like that in Iraq," the general told massed ranks of reporters in Baghdad's fortified Green Zone. "Military action is necessary to help improve security... but it is not sufficient."
Political talks involving all groups, including militants opposed to Prime Minister Nuri al-Maliki's government "will determine in the long run the success of this effort," he said.
Petraeus said one of five additional army brigades was already in place, another was partially deployed and the rest -- along with two extra battalions of marines -- would be in country by June.
Asked about reports that his second in command, General Raymond Odiernom, believed that the extra troops would be needed until early 2008, Petraeus said the timeframe had yet to be decided.
"I think you generally think that if you're going to achieve the kind of effects that we probably need, I would think it would need to be sustained certainly some time well beyond the summer," he said.
US troops, he said, will work with Iraqi colleagues to enable "the Iraqi people to control the demons responsible for the vicious sectarian violence of the past year -- demons that tore at the very fabric of Iraqi society."
"Indeed, our operations will endeavour to provide Iraq's citizens and leaders with the chance to mend that fabric."
Petraeus took command of US-led forces on February 10 after US President George W. Bush admitted two operations launched last year to pacify Baghdad had failed because of a lack of troops to hold cleared areas.
He must now oversee a strategy to contain faction fighting between Iraq's Sunni and Shiite factions and thus give Prime Minister Maliki's Iraqi government time to pursue a programme of national reconciliation.
His task will not be easy, with many Iraqis opposed to the US military presence in their country.
With Karbala flooded with pilgrims, Iraqi security forces were taking no chances and the town's governor Aqil Al-Khazali said tight security was in place, including modern equipment to detect explosive vests.
Outside the city, however, attacks continued, albeit without the savagery of recent days: six Shiite pilgrims were wounded in separate bomb and gun attacks in towns south of Baghdad as they walked towards Karbala, police said.
Security officials said six people were killed in other parts of the country on Thursday, including two policemen who died when a car bomb exploded in the northern city of Mosul.
Iraqi forces have recaptured 42 of the 140 prisoners who escaped when Al-Qaeda militants stormed a jail outside the northern city of Mosul, security officials said on Thursday.
Brigadier General Mohammed Al-Wagaa, who runs the Mosul police operations room, said the fugitives had been rounded up in various regions of the city since Tuesday's brazen attack by heavily-armed insurgents.
Five Iraqi army soldiers were killed by a roadside bomb near Tuz Khurmatu in northern Iraq, Colonel Abbas Hamed Amin said.
Security officials said at least six people were killed in other parts of the country on Thursday, including two policemen who died when a car bomb exploded in the northern city of Mosul.
Democrats demand withrawal from Iraq
US Democrats directly challenged President George W. Bush Thursday, with a plan to withdraw troops from Iraq by late 2008, and much sooner if progress is not made in the violence-wracked country.
Democratic leaders in the House of Representatives said their proposal would refocus US resources on the war on terror in Afghanistan.
The bill requires Bush to certify Iraq has made meaningful and substantial progress in meeting benchmarks by July 1, and October 1, this year.
If those benchmarks are met, troops must begin immediate redeployment on March 1 next year and the withdrawal must be complete within 180 days.
In the unlikely event that Bush certified no progress was being made in Iraq on July 1, or October 1, the redepolyment would have to begin immediately.
It was not clear whether the plan, the long-awaited response to Bush's war strategy, which will face significant opposition from the White House, Republicans in Congress, and even some members of the Democratic rank and file, would have enough votes to pass through the House.
Blair blasts Britons over Iraq war