U.S. says Iraq forces could control provinces this year

"(It) remains my hope that the pace of the drawdowns in the second half of the year will be what it was in the first half of the year," he said.

Lt. Gen. James Dubik, commander of security transition in Iraq, told the U.S. House of Representatives Armed Services Committee on Thursday that the number of Iraqi security forces, or ISF, could exceed 580,000 by the end of the year, up from the current 500,000.

But he also expressed caution about their abilities.

"Force structure and capability still lack a certain maturity. The ISF have not yet achieved self reliance in all area of logistics, maintenance and life support," he told the panel.

Odierno said he was confident the withdrawal of the five brigades will occur despite expectations for an upswing in insurgent attacks as militants respond to a new joint offensive known as Operation Phantom Phoenix.

The operation has killed or captured 92 "high-value individuals, according to the U.S. military.

"While we may see a short-term increase in violence in response to our operations in the weeks ahead, I expect Phantom Phoenix to contribute significantly to the population's security," Odierno said.

Gates' hopes for further reductions in U.S. forces this year will depend on a March assessment by the U.S. commander in Iraq, Gen. David Petraeus.

"To predict now whether we can go lower or not is difficult and I would not want to make that prediction right now," Odierno said.