More than 18,000 extra U.S. troops have been deployed around Baghdad as part of the campaign, which began in mid-February and is seen as a last-ditch attempt to drag Iraq back from the brink of all-out sectarian civil war.

The last of five brigades to be deployed in the crackdown will be in place soon, military spokesman Lieutenant-Colonel Christopher Garver said, adding that it would not be possible to judge the crackdown's success until all units were in place.

"Obviously we're constantly doing an assessment of the plan, but that plan doesn't kick in until everyone's here. We control about a third of the neighbourhoods," he said.

But with violence spiking across Iraq as Sunni Islamist al Qaeda and other militant groups try to derail the crackdown, U.S. President George W. Bush and military commanders have warned that a bloody summer lies ahead.

"It's going to get harder before it gets easier," Garver said of the counter-insurgency effort. "We know it's going to be a tough fight over the summer."

The New York Times reported earlier on Monday that according to an internal military assessment and local commanders, U.S. and Iraqi troops controlled 146 of Baghdad's 457 neighbourhoods.

The newspaper said the one-page military assessment showed that troops had either not begun operations or still faced resistance in the capital's remaining 311 neighbourhoods.

It said military planners had hoped most Baghdad neighbourhoods would be under control by about July. "We were way too optimistic," the paper quoted one unidentified officer as saying, adding that September was a more realistic goal.

"SNAPSHOT"

"That's kind of like a snapshot, those numbers are going to change the more you're working through those neighbourhoods," Garver told Reuters. He said he had not seen the military assessment.

Bush won a bruising battle with Congress for war funding but is under growing pressure, including from within his own Republican Party, to show progress in the unpopular war or start bringing troops home.

The crackdown in Baghdad and other areas is meant to buy time for Prime Minister Nuri al-Maliki's government to meet a set of political targets set by Washington aimed at promoting national reconciliation.

June is showing similar casualty rates to May, with 17 U.S. soldiers killed in the first three days, 14 of them reported on Sunday.

May was the third-worst month for U.S. soldiers since the invasion to topple Saddam in March 2003. The worst months were November 2004, when 137 were killed, and April the same year when 135 died.

Garver said more casualties were anticipated during the crackdown because thousands more troops were moving out of bases into command outposts, leaving them more exposed to attack.

"There is a short-term risk and that is what we've anticipated would happen in bringing about long-term benefit," Garver said.

Tens of thousands of Iraqis have been killed in the same period. While there was a significant reduction in the number of targeted sectarian killings early on in the security crackdown, those numbers have risen again, with dozens of bodies being found in Baghdad almost every day.