U.S. forces backed by tanks and aircraft pounded positions of a Shiite Muslim militia in the Iraqi shrine city of Karbala early today, killing at least 18 fighters who had been using holy sites as cover, the U.S. military said.

In Baghdad, the military announced the arrest of four persons in connection with the brutal murder of an American civilian, Nicholas Berg, who was beheaded by masked captors in a video that was posted on the Internet. Two of the four were subsequently released, and the other two were being questioned to determine whether they had any knowledge of the murder or involvement in it.

The military also announced the release of 454 detainees from Abu Ghraib prison, the scene of abuses by U.S. military guards, and said nearly 400 others are scheduled to be freed at the end of the month.

Later, the U.S-appointed Iraqi Governing Council condemned a raid on the home and offices of Ahmed Chalabi, a council member and longtime U.S. ally, by Iraqi police with support from U.S. troops and civilian advisers. The council said it would demand an explanation of Thursday's raid from the U.S. occupation authority in Iraq.

In Washington, the nation's top general defended the military's intelligence relationship with Chalabi's organization, which has received millions of dollars in U.S. funding over the years, including monthly payments of $335,000 to its intelligence arm that were recently halted.

Air Force Gen. Richard B. Myers, chairman of the Joint Chiefs of Staff, told a House Armed Services Committee hearing that U.S. military intelligence in Iraq appreciated the information it had received from Chalabi's Iraqi National Congress.

"That intelligence was accurate and useful in many cases," Myers said in response to questioning from skeptical Democratic congressmen. Asked by Rep. Jim Cooper (D-Tenn.) about allegations that Chalabi may have been an agent for Iran and "may have deliberately misled our nation for years," Myers said he knew only that Chalabi's organization "has provided intelligence . . . that saved soldiers' lives."

Asked "what changed" in the U.S. relationship with Chalabi, who had been known as the Pentagon's man in the Iraqi opposition to former president Saddam Hussein, Myers said, "I can't answer that."

In response to questioning from Rep. Tim Ryan (D-Ohio) -- who asked, regarding Chalabi, "Have we been duped by a con man?" -- Myers said, "I don't have the information that would allow me to make that judgment." He added, "People in the Iraqi National Congress have provided very good intelligence to our forces in Iraq that have prevented our forces from dying." He said he would be prepared to talk about Chalabi's relationship with Iran in a closed session.

At a press briefing in Baghdad, Army Brig. Gen. Mark Kimmitt, the chief U.S. military spokesman in Iraq, said U.S. soldiers were involved in the raid on Chalabi's offices only to provide an "outer cordon" to maintain security for the Iraqi police and to prevent any "collateral violence."

Daniel Senor, a spokesman for the Coalition Provisional Authority, denied that any plainclothes personnel from the CIA, FBI or Defense Intelligence Agency participated in the raid. He said private American contractors who work for the Iraqi Interior Ministry were present "to observe and advise the Iraqi police during this operation, as they do on numerous operations." He said an American woman who identified herself as an employee of the Iraqi National Congress was also on the scene.

The American contractors, Senor said, "were not there at the direction of the coalition."

Kimmitt said U.S. forces in Karbala, burial place of Hussein, the grandson of the prophet Muhammad, came under heavy fire from rebel militiamen as the American soldiers were covering a withdrawal from the Mukhayam mosque in the center of Karbala. Kimmitt said the militiamen fired rocket-propelled grenades from a building directly across the street from the city's main holy shrine.

The U.S. soldiers called in an AC-130 gunship to direct precise fire against the militia position to avoid "any collateral damage on the holy shrines," Kimmitt said. He said the AC-130 was used again in a "sustained engagement" in the northern part of the city, where militiamen directed "multiple RPG attacks" against U.S. troops in the afternoon.

The rebel fighters belong to the Mahdi Army, a militia loyal to a radical young Shiite cleric, Moqtada Sadr, who has been inciting resistance to the U.S. occupation from strongholds in the Shiite holy cities of Najaf and Karbala. U.S. authorities contend that many residents of the cities oppose Sadr and want U.S. forces to put down his militia.

Kimmitt said the four arrests in the Berg case were carried out by U.S. forces on the basis of "Iraqi tips." He said of the two persons still in custody: "We're questioning them. We may find out that they have no association with the murder, but we will continue to question them for some period of time, until we're convinced they're innocent." He said he had no other information on the suspects.

"We have some intelligence that would suggest they had knowledge, perhaps some culpability, but we're not going to know until we've actually finished the questioning," Kimmitt said.

Rep. Duncan Hunter (R-Calif.), the chairman of the House Armed Services Committee, said he called today's hearing to focus on the conduct of the war in Iraq, which has been largely overshadowed by a scandal over the mistreatment of Iraqi prisoners by U.S. military police at the Abu Ghraib jail west of Baghdad.

"While the media has been absorbed with the misdeeds of a few MPs, our soldiers turned back an uprising by Moqtada al-Sadr and his Iranian-supported Mahdi brigade," Hunter said in an opening statement. "He and his militia are now cornered and have retreated into holy sites in Najaf, proving to Iraqis and the world that he has no respect for Islam or Iraqi's future as a nation."

Myers told the committee that he does not expect an Iraqi interim government, scheduled to assume political power June 30 from the Coalition Provisional Authority, to ask U.S. forces to leave the country. Secretary of State Colin L. Powell has said that if asked to depart by the interim government, the United States would comply.

"Iraqis understand that security has to be a partnership for some time," Myers said. "I think we are going to be wanted. I think we are going to be asked to stay." He said the worst fear of many Iraqis is that U.S. forces would leave, plunging the country "into anarchy and chaos."

While violence in Iraq may escalate and "many more" people are likely to be killed, Myers said he was optimistic about the U.S. mission there.

"I don't think we're on the brink of failure," he said in response to a question. "I think we're on the brink of success."