The new coalition-trained Iraqi police force is being infiltrated by insurgents, a US army general has said.
Tens of thousands of recruits have been trained
Maj Gen Martin Dempsey said about 10% of new officers were rebels and a further 40% had left their jobs - but the rest "stood tall and stood firm".
His comments came hours after a series of bomb blasts in southern Iraq killed at least 68 people.
In the city of Falluja, coalition forces say 36 insurgents have died in fighting which began on Wednesday.
US troops said this began when up to 60 rebels attacked them with small arms and rocket-propelled grenades - despite a ceasefire agreed earlier in the week.
The city, west of Baghdad, is held by Sunni militants.
Gen Dempsey, commander of the US army's 1st Armored Division, told media executives in an interview that he believed popular support for the coalition among Iraqis remained high, though it could not be taken for granted.
But he acknowledged that one in every 10 of the Iraqi security forces trained by the coalition ended up working against the US-led forces.
"About 50% of the security forces that we built over the past year stood tall and stood firm," he told the annual meeting of the Associated Press news agency.
Gen Dempsey said some Iraqis found it hard to accept the violence
"About 40% of them walked off the job because they were intimidated and about 10% actually worked against us."
Gen Dempsey, who commands the units in charge of Baghdad, said he believed the attacks in Basra may have been ordered to grab the headlines while other areas of Iraq were relatively calm.
A series of apparent suicide bombings targeted three police stations in the southern city which is under the command of UK forces.
Many of the dead and injured were children travelling in passing buses on their way to school in Wednesday's morning rush hour.
A fourth attack south of Basra is said to have killed three Iraqis and wounded five UK soldiers.
UK Foreign Minister Jack Straw has said the attacks would not delay the planned 30 June transfer of sovereignty back to the Iraqi people.
He also said he expected agreement to be reached on a new United Nations Security Council resolution, probably in May.
British officials do not think local Shias were responsible for the explosions, but blame them on "al-Qaeda type elements or former regime loyalists".
Briefing reporters in London, one official said: "The Shias have broadly accepted the British presence in Basra and I do not think this has changed."
In Washington, the Pentagon's top general said the increased violence and extended US troop deployments was pushing the cost of the war over budget.
"When the service chiefs last talked about this, there was, I think, a $4bn shortfall," Gen Richard Myers, chairman of the Joint Chiefs of Staff told the House Armed Services Committee.
"We thought we could get through all of August. We'd have to figure out how to do
The White House has said it does not expect to seek more money from Congress until early next year. But on Wednesday it did not rule out the possibility of asking for more funds before the end of the year.
US costs in Iraq are running at about $4.7bn a month, officials say.