The Senate will hold hearings soon into an alleged Marine execution of Iraqi civilians, a committee chairman said Tuesday as Republicans and Democrats voiced serious concerns about the accusations.

"We've got our duty and we'll do it. We'll go wherever the facts lead," Sen. John Warner, the Senate Armed Services Committee chairman, said of the allegations that U.S. Marines killed up to 24 Iraqi civilians last November in the town of Haditha without cause and then engaged in a cover-up.

The Virginia Republican told reporters he wanted to hold the first of what he expected to be a series of hearings on the allegations as soon as Army Maj. Gen. Eldon A. Bargewell finishes his investigation into whether military personnel lied about what occurred at Haditha. Warner said Bargewell likely would be the first witness called.

Congressional aides say Bargewell's report could be completed within the week but that the military's separate criminal investigation is not expected to be finished anytime soon.

In a letter to Defense Secretary Donald H. Rumsfeld on Tuesday, Warner requested the Pentagon tell him "the earliest possible date that the department could provide witnesses" for the first hearing. Warner stressed that delaying the results of such investigations will mean that "a mixture of information, misinformation and unconfirmed facts will continue to spiral in the public domain."

"We must do everything possible to prevent our military personnel, and indeed the values of all our military services, from being judged on unofficial information in the court of public opinion prior to the conclusion of official investigations," Warner wrote.

His counterpart in the House, Rep. Duncan Hunter, R-Calif., also has pledged to hold hearings in the House Armed Services Committee. But he, too, has said he wants to wait until the military completes its investigations to ensure Congress doesn't interfere with potential criminal proceedings.

Over the past week, more details have emerged about the November incident and a senior U.S. defense official said military investigators have evidence that points toward unprovoked murders by Marines.

Politically, the alleged murders in Haditha — as well as unsubstantiated claims of unprovoked civilian deaths by U.S. troops elsewhere in Iraq — threaten to further drag down support for the Iraq war and could turn into a political problem for President Bush and Republicans five months before congressional elections. Polls show voters increasingly uncomfortable with the direction of the war as well as more inclined to see Democrats leading Congress.

Returning to Washington after a weeklong Memorial Day break, lawmakers expressed grave concerns about Haditha and said the allegations could further tarnish the U.S. image abroad and hurt efforts to curb terrorism worldwide.

But Republicans and Democrats alike in both the House and the Senate also urged caution, saying that Congress and the public still didn't have all the facts and shouldn't prejudge the outcome of the military investigations.

"While it's too early to pass judgment on what happened, things don't look good," said Sen. James Inhofe, R-Okla.

Added Sen. Ben Nelson, D-Neb.: "If the initial reports are true ... clearly it's an atrocity."

"What we do know is pretty ugly stuff," said Rep. Adam Putnam, R-Fla., who returned from Iraq just days ago. "I fear that Haditha can have even greater negative consequences in perception than Abu Ghraib," where U.S. soldiers abused Iraqi prisoners in 2004.

Rep. C.A. "Dutch" Ruppersberger, D-Md., who also recently returned from Iraq, said that if substantiated, what happened at Haditha "hurts our reputation in the world."