Washington ? The U.S military will begin pulling out the additional troops it sent to Iraq as part of the so-called surge next spring and will have completed their withdrawal by next August, the No. 2 American commander in Iraq said Friday.

Lt. Gen. Raymond Odierno also said that Shiite factions now are producing nearly as much violence in Iraq as the Sunni extremist group al-Qaida in Iraq. When the surge began, the military said al-Qaida in Iraq was responsible for most of the attacks.

Odierno’s comments likely reflect the thinking of Gen. David Petraeus, the top military commander in Iraq, who’s preparing a highly anticipated report to Congress on the progress of U.S. strategy in Iraq. That report must be delivered by Sept. 15.

“The surge, we all know, will end sometime in 2008, in the beginning of 2008,” Odierno told reporters in a teleconference. “We know that the surge brigades will leave at 15 months, so that will be somewhere between April and August of ’08 when those units will leave based on the 15-month rotation.”

Odierno said it was possible Petraeus could ask for new troops to replace those units as they depart, but that “right now, our plan is not to backfill those units.” He said the 28,000 troops that make up the surge had taken five months to deploy and would take about that long to bring home.

Military officials have acknowledged that maintaining the surge after spring would be impossible under current Pentagon regulations requiring that troops be given one year at home between combat deployments.

Odierno didn’t comment on what might happen after the additional U.S. forces leave. U.S. officials have claimed that the surge plan has cut violence, but they’ve also expressed concern that the violence could resume once U.S. troops are reduced because there’s been no attempt at reconciliation between rival Sunni and Shiite groups.

Prime Minister Nouri al-Maliki’s government is floundering and shows no signs of passing legislation on the nation’s most divisive issues, including how to distribute oil revenue and when to hold provincial elections. The Bush administration had said that a reduction in violence from the surge would enable Iraqi legislators to deal with such issues and build a government that could hold Iraq together.

Instead, military officials in Baghdad fret that as soon as they pull back from the capital, the violence will return, just as it did after the U.S. pulled back from its last Baghdad operation nearly a year ago.

Odierno said the longer U.S. forces work with their Iraqi counterparts, the more apt they will be able to retain control of neighborhoods.