Iraq's embattled prime minister defended his government Sunday against American critics, saying they underestimate the problems facing this country and fail to appreciate his achievements "such as stopping the civil and sectarian war."

Criticism of Prime Minister Nouri al-Maliki's leadership has been growing in the run-up to this month's series of reports to Congress on political and security progress since President Bush dispatched nearly 30,000 more American troops to Iraq.

Sen. Hillary Rodham Clinton and Sen. Carl Levin, chairman of the Senate Armed Services Committee, have called for al-Maliki to be replaced.

"Regrettably these statements made by U.S. officials sometimes exceed reasonable limits and at the same time send regrettable messages which help terrorists think that the security situation in the country is weak and the political forces are not cohesive," al-Maliki told reporters.

He added that critics are sending "negative messages that encourage terrorism."

"Maybe they don't know the size of the destruction that Iraq passed through and the big role of the Iraqi government and its achievements such as stopping the civil and sectarian war," al-Maliki said.

During an interview broadcast Sunday by Iraqi state television, U.S. Ambassador Ryan Crocker also urged patience with the Iraqis as they try to reach power-sharing agreements among Sunnis, Shiites and Kurds.

"After 35 years of injustice under Saddam Hussein, there are some problems since liberation and the problems of 40 years cannot be solved in a year or two," Crocker said, speaking in Arabic. "What is important is that there is progress."

A draft report still under review at the U.S. Embassy in Baghdad includes allegations that the al-Maliki government is riddled with corruption and has, in some cases, sought to derail or prevent investigations into alleged graft by Shiite-controlled agencies or allied officials, according to two U.S. officials familiar with the findings.

The draft report by the Office of Accountability and Transparency has yet to be reviewed by senior diplomats who are members of an anti-corruption group or by Crocker, who along with top commander Gen. David Petraeus is due to report to Congress during the week of Sept. 10.

Al-Maliki's boast of "stopping" the war may fall short of the facts, but it reflects the frustration of Iraqi officials who believe congressional critics are pushing for political goals that are unrealistic given the depth of Iraq's political and social divisions.

Those divisions exist not only between Sunnis and Shiites but within each of those sectarian communities.

In a bid to ease divisions within Shiite ranks, al-Maliki ordered what he promised would be an "unbiased investigation" into last week's deadly fighting between rival Shiite militias during a religious festival in Karbala. Up to 51 people were killed in two days of clashes, officials said.

The clashes escalated following a confrontation between supporters of anti-American cleric Muqtada al-Sadr, head of the Mahdi Army militia, and mosque guards from the Badr Organization, affiliated with the country's biggest Shiite party, the Supreme Islamic Iraqi Council.

Al-Sadr, who has denied that the Mahdi Army provoked the confrontation, announced a surprise six-month suspension of the militia's activities last Wednesday following the fighting in Karbala, in an apparent attempt to deflect criticism and rein in extremists in the ranks.

Al-Sadr demanded an investigation into the two-day clashes that broke out Monday night in Karbala. His followers claimed that local officials from the Supreme Council have used the Karbala clash as a pretext to crack down on their political rivals in the al-Sadr movement.

Brig. Gen. Raid Shaker, commander of Karbala police, said 300 detainees are being questioned over the Karbala incident.

Al-Sadr's office said in a statement that more than 200 of them were al-Sadr loyalists, making al-Maliki's praise of the decision to freeze the Mahdi Army nothing more than "ink on paper."

"After the procrastination we had seen in the past two days, we warn the Iraqi government and the executive authorities in Karbala if they don't open a fair, neutral and quick investigation, the Sadr office will be obliged to take unspecified measures," the cleric's spokesman Sheik Salah al-Obeidi said in Najaf.

Jawad al-Hasnawi, a Sadrist member of Karbala's provincial council, accused the prime minister of reneging on promises to stop detaining people in the Karbala violence.

"They have taken us back to the era of the former dictatorship," al-Hasnawi said.

Also Sunday, a car bomb exploded on Eddin Square in northern Baghdad, killing nine people and injuring 15, police said on condition of anonymity because they were not authorized to release the information.

U.S. troops killed eight extremists and arrested four in a series of raids in Baghdad, the military said. Troops freed nine Iraqi hostages during the operation, the command said.

A suicide driver detonated his truck at an Iraqi army checkpoint near Taji, just north of Baghdad, killing one civilian and six other people, three of them soldiers, police said on condition of anonymity because they were not supposed to release the information.

U.S. military spokesman Rear Admiral Mark Fox confirmed Sunday that U.S. forces arrested six people in a raid Thursday at the government newspaper Al-Sabah in Baghdad.

Fox said the military staged the raid on "actionable intelligence" and found illegal weapons when they searched the facility.