Defense Secretary Donald H. Rumsfeld said Wednesday that he couldn't promise that a significant number of U.S. forces would return from Iraq by year's end.

Pressed by lawmakers, the Pentagon chief said he hoped for a large American troop withdrawal this year but could not assure them that would occur.

"No. No one can," Rumsfeld told a Senate committee. Still, he said. "It's obviously our desire and the desire of the troops and the desire of the Iraqi people."

Testifying alongside Rumsfeld, Gen. Peter Pace, the chairman of the Joint Chiefs of Staff, said troops cannot withdraw completely from any of Iraq's 18 provinces within the next three months, even though most are considered calm.

On the other side of Capitol Hill, Rep. John Murtha said a Pentagon war crimes investigation will show Marines killed more than a dozen innocent Iraqi civilians "in cold blood" in the town of Haditha.

Officials have said 15 Iraqis, eight insurgents and a U.S. Marine were killed during a Nov. 19 firefight that began when a roadside bomb detonated next to a joint Iraqi-U.S. squad patrolling Haditha.

"Our troops overreacted because of the pressure on them and they killed innocent civilians in cold blood. And that's what the report is going to tell," Murtha, D-Pa., told a news conference at which he was making the point that U.S. forces are "overstretched and overstressed" by the war.

The Pentagon said late Wednesday that it had taken "immediate and aggressive action" to look into the charges after they surfaced. "It would be premature to judge any individual or unit until the investigation is complete," said Defense Department spokeswoman Cheryl Irwin.

A highly decorated former Marine and now a leading House critic of the Iraq war, Murtha also renewed his appeal for a quick pullout from Iraq, saying the situation had deteriorated there in the six months since he introduced a resolution that would have ended U.S. military presence.

Lawmakers from both political parties are anxious to see U.S. troops come home, particularly as they face a congressional election this fall. Polls show that their constituents largely don't support the war as both casualties and costs climb.

The Republican-led Senate demonstrated its sentiment last year for a reduced U.S. role by voting that 2006 should be a year of major transition in Iraq.

Since the March 2003 invasion, U.S. forces have dropped from a high of 160,000 during Iraqi elections to about 132,000.

More than 2,400 American soldiers have died in Iraq. The nonpartisan Congressional Research Service estimates the conflict has cost $261 billion so far, not including Bush administration requests for tens of billions more for this year and next.

Lawmakers on the Senate Appropriations Committee's defense panel — in a bipartisan signal of unease — sought updates from Rumsfeld and Pace on the status of training and equipping Iraqi security forces.

"I think we would all agree — everyone in this room — that the quickest ticket out is the adequacy of the Iraqi military and police," Sen. Mitch McConnell, R-Ky., said.

Democrats, in particular, pressed Rumsfeld and Pace on the prospects for scaling back U.S. forces in Iraq and several cast doubt on administration claims of progress in training Iraqi forces.

"The American people want to know when our forces currently in harm's way in Iraq are going to be out of harm's way, redeployed to a safe location outside of Iraq," Sen. Dick Durbin, D-Ill., said. "Can you tell us that before the end of this calendar year a significant number of American troops will be redeployed out of harm's way in Iraq?"

"I can't promise it," Rumsfeld said. "We would hope so."

As he usually does, Rumsfeld said President Bush will make the final decision on when to withdraw forces after military commanders recommend reducing forces based on conditions in Iraq. Those include the ability of Iraqi forces and continued political progress in Baghdad.

Both Rumsfeld and Pace said Iraqi forces are making progress and increasingly taking the lead from U.S. and coalition forces as the new Iraqi government takes shape.

Pace said 14 of the 18 Iraqi provinces are "essentially calm, secure."

Sen. Patrick Leahy, D-Vermont, asked if it is possible for U.S. troops in the next three months to withdraw completely from any of those 14 provinces.

"No, sir," Pace answered, saying U.S. forces still are needed to assist Iraqi forces with logistics.

The testimony was Rumsfeld's first on Capitol Hill since several retired generals publicly criticized him over the war and in some cases called for his resignation.

Asked about the criticism, Rumsfeld said it should not be a surprise, given the kinds of pressure he has put on civilian and military leaders to change.

The hearing was briefly interrupted by a woman wearing a hot pink T-shirt that said "Stop The War Now" and bore the number 2,450, an apparent reference to the number of U.S. forces who have died in Iraq.

"Liar," she shouted at Rumsfeld. A uniformed officer escorted her out without a struggle.