WASHINGTON (CNN) -- About 8,000 of the 30,000 "surge" troops sent to Iraq in 2007 will not go home as planned this summer, the Pentagon said Monday.

President Bush ordered nearly 30,000 additional troops to Iraq in January 2007 as part of a troop surge.

Support troops -- including helicopter crews, supply units, military police and headquarters staff -- will still be needed when the additional combat units return home, said Lt. Gen. Carter Ham, the chief of operations for the Joint Chiefs of Staff. In particular, U.S. troops are still needed to guard Iraqi prisons, he said.

"The transfer of responsibility for detention operations has not progressed as rapidly as we would like to the Iraqis, so there's a need to have that force sustained, as well," Ham said.

President Bush ordered nearly 30,000 additional troops to Iraq in January 2007 to pacify Baghdad and its surrounding provinces. When the last of the five Army combat brigades and two Marine battalions ordered in as part of that campaign leave Iraq by July, 140,000 troops will remain -- about 8,000 more than the 132,000 U.S. troops stationed there before the surge, Ham told reporters at the Pentagon.

"This will be very much conditions-based, but that's our projection as of today," he said.

He said top U.S. commanders are debating whether to make further withdrawals after July. The U.S. commander in Iraq, Gen. David Petraeus, is scheduled to issue another report on the progress of the war in April, and Ham said it would be "premature" to discuss further withdrawals now.

The Bush administration has touted a sharp reduction in the sectarian warfare that wracked Baghdad throughout 2006 -- and an accompanying drop in American death tolls -- as signs of the campaign's success. But Ham said the support troops are still needed to preserve that progress.

"Rather than look at this negatively, I would say there is an opportunity now to take advantage of the security that has been established by the five surge brigades," he said. "And you want to sustain that and not jeopardize the gains that have been achieved."

Critics say the political goals of the surge -- an Iraqi settlement of the nearly 5-year-old war -- have not yet been accomplished. Sen. Chuck Hagel, R-Nebraska, said Sunday that Iraq remains far from stable.

"If we've had so much success, then why are our commanders concerned about pulling troops down and wanting to freeze them at at least the level that we had before the surge?" Hagel told CNN's "Late Edition."

But Ham said the campaign has encouraged Iraqis to take steps toward reconciliation and to turn against Islamic jihadists loyal to al Qaeda, which is blamed for some of the war's worst attacks on civilians.

"I would say that there is increasing pressure on al Qaeda in Iraq everywhere inside Iraq," he said. "Again, it is premature to declare victory or anything, but it is very clear that, wherever al Qaeda in Iraq tries to operate, they are increasingly being resisted by Iraqi security forces, clearly by the U.S. and other coalition members, and most importantly by the people of Iraq -- and that's really what's making the difference." E-mail to a friend

All About Iraq • George W. Bush • Baghdad • Al Qaeda in Iraq