WASHINGTON (CNN) -- U.S. troops could begin leaving Iraq by the end of the year because this summer's troop "surge" has improved security, the top U.S. commander in Iraq told Congress Monday.
General David Petraeus said U.S. forces could be reduced to pre-surge levels by next summer.
Petraeus and U.S. Ambassador to Iraq Ryan Crocker are testifying at the invitation of Congress this week in an effort to inform lawmakers of the war's progress.
Their reports were not reviewed by the Bush administration.
President Bush will base his mandated report to Congress about war strategy on Petraeus' and Crocker's recommendations. A senior administration official said Friday that it is "very likely" Bush will speak to the nation about Iraq in a prime-time address from the White House this week.
President Bush ordered nearly 30,000 additional troops to Iraq inJanuary as part of a campaign to pacify Baghdad and its surrounding provinces and stabilize the Iraqi government.
Petraeus said he has recommended the withdrawal of a Marine expeditionary unit later this month and a brigade combat team in December, plus the "reduction of eight further brigade teams and the battalion combats in 2008 until we reach the pre-surge levels of 15 brigade combat teams by mid-July 2008."
But Petraeus said he would not speculate on force reductions beyond next summer.
"Our experience in Iraq has repeatedly shown that projecting too far into the future is not just difficult, it can be misleading and even hazardous," he said.
After several days of questions whether Petraeus would be influenced by the Bush administration's line on Iraq, Petraeus in his opening statement said the testimony was his alone.
Petraeus said coalition forces had seen good success against al Qaeda in Iraq, knocking it "off-balance" and pursuing its leaders relentlessly.
The general said that tribes and local citizens have rejected al Qaeda and other insurgents, especially in the volatile Anbar province.
The general called this the "most significant development in the past six months."
Depsite successes, "the level of civilian deaths is still too high," Petraeus said.
Before Petraeus began to testify, the head of the House Armed Services Committee challenged Petraeus and Ambassador Ryan Crocker to convince him that the war effort in Iraq is worth continuing.
Rep. Ike Skelton, D-Missouri, told Petraeus and Crocker that he was particularly curious whether reconciliation among warring Iraqi sects is imminent.
"I hope, General Petraeus and Ambassador Crocker, that you can persuade us that there is substantial reason to believe that Iraq will turn around in the very near future," Skelton said.
Rep. Duncan Hunter, R-California, warned Petraeus that he would face hostile skepticism from Democrats.
"The last week or so has been spent attacking your credibility, with major attacks here in the United States, some of them emanating from right here" in Congress, Hunter said.
Capitol Police escorted protesters out of the hearing room where Petraeus was testifying, and arrested at least four people, including anti-war protester Cindy Sheehan.
Before the hearing began, Democrats and Republicans exchanged statements over a full-page ad in the New York Times in which the liberal advocacy group MoveOn.org accused Petraeus of "cooking the books for the White House" and saying "General Petraeus is a military man constantly at war with the facts." Read how Moveon.org ad draws fire
The White House and Republican congressional leaders demanded that Democratic leaders distance themselves from the ad. White House spokesman Tony Snow called it a "boorish, childish, unworthy attack."
And referring to congressional critics of Petraeus' independence, Senate Minority Leader Mitch McConnell said, "I also resent the comments of those who sat comfortably in their air-conditioned offices thousands of miles away from the firefights and roadside bombs and tried their Washington best in recent days to impugn the general's good name."
Petraeus will appear before the Senate Foreign Relations Committee on Tuesday.
In his opening statement, Crocker praised the Iraqi leadership for their efforts to form a workable government in the country.
"Iraq's leaders have the will to tackle the country's pressing problems," he said.
Crocker gave particular praise to leaders at the provincial and local levels for their work at reconciliation.
The ambassador said Monday that the Iraqi economy is recovering but still "performing significantly under potential" because of power outages and because insecurity in rural areas is raising transportation costs, especially in the manufacturing and agriculture sectors.
"We must acknowledge that 2006 was a bad year in Iraq," Crocker said.
But this year's troop surge had made a difference, he said.
"We have give Iraqis the time and space to reflect on the king of country they want," he said.
"I cannot guarantee success in Iraq," Crocker said. "I am certain that abandoing our efforts will bring failure," adding that it could leave a vacuum which Iran would seek to fill.
Sen. Dianne Feinstein, D-California, told "Fox News Sunday" that Petraeus' position of authority in Iraq clouds his view.
"General Petraeus is there to succeed," Feinstein said. "I don't think he's an independent evaluator."
A CNN/Opinion Research Corp. poll taken last month found about two-thirds of Americans -- 64 percent -- oppose the Iraq war, and 72 percent say even if Petraeus reports progress, that won't change their opinion.
The poll also found a great deal of skepticism about the report, with 53 percent saying they do not trust Petraeus to give an accurate assessment of the situation in Iraq.
Iraqi Prime Minister Nuri al-Maliki said Monday his country's security forces were not ready to stand alone.
"Despite the security improvement, we still need more efforts and time in order for our armed forces to be able to take over security control in all Iraqi provinces from the multinational forces," he told Iraq's parliament.
U.S. military data obtained by CNN indicates that 165 Iraqis were murdered in Baghdad last month, a slight increase from the previous two months. However, the number represents a significant decrease since the Baghdad security plan began earlier this year.
It is not clear how the U.S. military obtained the number, but CNN statistics -- compiled from numbers released by the Iraqi Interior Ministry -- suggest 428 Iraqis were murdered in Baghdad in August, their bodies dumped in the streets. In July, 612 Iraqis were murdered, according to the Interior Ministry.
A U.S. military chart indicates monthly casualties in Baghdad, which spiked in November at 2,200, dropped to 980 last month. The chart does not break the casualties down into deaths and injuries.
The military data focus only on Baghdad and do not address the increase in violence in other parts of the country since the Baghdad security plan kicked off in February.
CNN's Barbara Starr, Dana Bash and Ted Barrett contributed to this report.