By David Morgan

WASHINGTON, May 31 (Reuters) - A September deadline for an official military assessment of President George W. Bush's troop buildup in Iraq could come too early to be a true measure of the strategy, the No. 2 commander in Iraq said on Thursday.

Army Lt. Gen. Ray Odierno also said the U.S. military hoped to reach cease-fire agreements with Iraqi insurgents, amid concern that violence could flair this summer as the so-called U.S. troop surge reaches its full momentum in Baghdad.

The top U.S. operational commander in Iraq told reporters that the full impact of the surge will likely come in August -- a month before the top U.S. commander in Iraq, Gen. David Petraeus, is to report on the effects of the 30,000-troop buildup aimed at securing Baghdad.

The September report is widely viewed as a watershed for the unpopular U.S.-led war, with Democrats and some Republicans saying they could back a troop withdrawal absent evidence of progress.

"Right now if you asked me, I would tell you I'll probably need a little bit more time to do a true assessment," Odierno said at a Pentagon news briefing where he appeared via video link from Iraq.

"I will provide my assessment to Gen. Petraeus who will then take that forward," he said. "If I think I might need a little more time, I will give an assessment saying that I'd like to have more time."

"That will be part of the assessment that I make," Odierno added.

He declined to say how much extra time he might need: "We still have ... 90 to 100 days until Labor Day." The U.S. Labor Day holiday is Sept. 3.

Bush and other U.S. officials have predicted heavy fighting this summer as the surge takes hold in Baghdad. Odierno said insurgents and al Qaeda are likely to see violence as their means of thwarting the strategy and forcing a pullout.

But he said U.S. commanders have been empowered to negotiate cease-fires and other agreements with insurgents and estimated that 80 percent of Sunni insurgents and Shi'ite militia members could be reconciled.

"We're talking about cease-fires and maybe signing some things that say they won't conduct operations against the government of Iraq or against coalition forces," Odierno said.

"It's happening in small levels ... it's just beginning, so we have a lot of work to do in this," he added.

The general said even some elements of al Qaeda in Iraq -- which U.S. officials have described as public enemy No. 1 there -- could be brought to the negotiating table.

"Very few of al Qaeda are reconcilable. But there might be a small portion," he said.

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