BAGHDAD — At least 17 Iraqis were killed Sunday night when U.S. and Iraqi special forces stormed a mosque and clashed with Shiite Muslim militiamen, police officials said, further inflaming the country as its leaders struggled to form a new government and stem sectarian violence.

An Iraqi police official said the dead were Shiite worshipers at the Mustafa mosque in northeast Baghdad. State-owned Al Iraqiya television showed more than a dozen male corpses, at least one of them elderly, laid out in what appeared to be a prayer room as a grieving man in white robes stepped among them on a blood-smeared concrete floor.

The incident is politically explosive because the mosque is a stronghold of followers of the radical Shiite Muslim cleric Muqtada Sadr, whose Iranian-backed movement has a powerful bloc in parliament and a large sectarian militia. Sunday's clash was the most serious between that militia and U.S. forces since Sadr led two anti-American uprisings in 2004.

In increasingly insistent language, U.S. Ambassador Zalmay Khalilzad has been pressing Iraq's leaders to disband such militias, which he blames for much of the sectarian killing here since the Feb. 22 bombing of a Shiite shrine in the mostly Sunni Arab city of Samarra.

Dozens of people are found dead each day in a shadowy campaign of executions.

On Sunday, police and medical officials in Baqubah got a tip that the bodies of 30 beheaded men had been found earlier in the day beside a highway between Baqubah and Baghdad. Iraqi army troops were waiting for U.S. military support before going into the insurgent-infested area to retrieve them.

If Iraqi accounts of the assault on the mosque are confirmed, it could undermine the U.S. effort to disband the militias and centralize control of security issues.

The early evening raid on the mosque, in the Ur district, came as security in and around Baghdad appeared to be deteriorating rapidly.

Witnesses told of helicopters buzzing overhead and heavily armed Humvees sealing off the streets around the mosque, which also serves as a headquarters for Sadr's militia. Soldiers tried to enter, setting off an hourlong gun battle that lasted past dark.

U.S. military officials, responding to inquiries about the incident, issued a report saying 16 insurgents had been killed in a raid about the same time by American and Iraqi forces in Adhamiya, a Sunni district. The statement said "no mosques were entered or damaged during the operation."

The two districts are about three miles apart, and the officials did not return telephone calls to clarify whether their report referred to a different operation.

Because an overnight curfew took effect before news of the clashes broke, it was impossible to travel to the area to reconcile the two accounts.

In a statement read on television, Prime Minister Ibrahim Jafari said he was "deeply concerned" by the killings, and had elicited a promise from Army Gen. George W. Casey, the top U.S. military commander in Iraq, to investigate them.

"We call upon the sons of our people to be aware of what is being plotted against the country," Jafari said. "We hope that they will have patience until the conclusion of the ongoing, immediate investigations."

Reaction from Sadr's followers was strident.

"I think we are going to have a firm stance against the American forces because of this crime," said Salam Maliki, a Sadr follower who is the interim government's minister of transport.

He blamed the raid on Khalilzad's vocal campaign against the militias.

The American operation also drew condemnation across Iraq's sectarian divide.

Saleh Mutlak, a leading Sunni politician, said: "We are trying to dissolve the militias and stop the bloodshed. This act by American troops will pull us into civil war."

One witness to the raid, shown speaking on state television, said the incident unfolded during funeral services at the mosque for a man killed three days earlier in an attack by Sunni-led insurgents.

Jawad Maliki, an official of the prime minister's Islamic Dawa Party, told the TV station that U.S. and Iraqi forces chased a wanted suspect to the mosque.

Hazim Araji, a Sadr representative in Baghdad, said people inside the mosque heard that American troops were outside before the shooting started. He said the Sadr movement contacted officials in the Iraqi government to ask what was happening.

"They told us we have to calm down the worshipers because they are not the targets," Araji said in a telephone interview. "We were told that the worshipers should stay inside."

The details of what happened next are unclear. There were varying accounts of what kind of weapons the troops used and whether they entered the mosque. One policeman, in an unofficial account, said the mosque appeared to have been hit by rockets and that part of it caught fire.

Abdul-Hadi Darraji, another Sadr representative, said the movement's office inside the mosque was damaged by flames.

Capt. Khattab Omer, a police official, said 17 people he described as worshipers were killed and four others were injured. Other accounts put the death toll as high as 22.

Residents of the neighborhood, contacted by telephone, said they could hear gunfire and ambulances. Black-clad gunmen of Sadr's Al Mahdi militia poured into the streets, they said.

The operation described by the U.S. command appeared to differ markedly. It was aimed, U.S. officials said, at the Sunni-led insurgency rather than a Shiite mosque. In scant detail, the U.S. command's statement described a joint U.S.-Iraqi raid on a "terrorist cell" that left 16 insurgents dead, one Iraqi soldier wounded, 15 men under arrest and a cache of bomb-making material seized.

The storming of the mosque came a few hours after at least one mortar round fell about 50 yards from Sadr's home in the southern holy city of Najaf, wounding a guard and a child.

Sadr blamed the shelling on U.S.-led forces. "Either they overlook these attacks or they do it themselves," he said in a written statement. "I urge my brothers not to be trapped by the Westerners' plots."

The young cleric's rising stature as a political player has alarmed U.S. officials hoping to wind down the American presence and leave behind a stable government.

U.S. and Iraqi officials worry that Sadr's movement, with its arsenal of weapons and radical ideology, poses a threat to any central authority and inspires other political movements to take up arms.

Prodded by Khalilzad, the five main political factions in the parliament elected Dec. 15 have been trying to form a unity government with a strong national army and police force.

They have made progress over the last week, but Sunday's talks hit a snag over a proposal that would allow the next prime minister, again expected to be a Shiite, to oversee the security forces. Sunni negotiators objected, charging that the current Shiite-controlled Interior Ministry is infiltrated by militias that have carried out extrajudicial killings of hundreds of unarmed Sunnis in recent months.

President Jalal Talabani, a Kurd, canceled a trip to this week's Arab League summit in Sudan to avoid any delay in the negotiations, which he has been leading.

Sunday brought more violence. In addition to the 30 bodies discovered near the highway, police said 13 corpses were found in other parts of the country and 10 other Iraqis were killed. They included a 13-year-old boy hit by a bomb blast as he walked to school in the southern city of Basra.

Arkan Bawi, a police major accused of heading a death squad in Baqubah, was summoned to Interior Ministry headquarters in Baghdad and placed under arrest. It was unclear what prompted the rare instance of an official investigation into such charges.

The accused man was identified as a brother of the police chief in Diyala province.

Times staff writer Raheem Salman and special correspondent Asmaa Waguih in Baghdad contributed to this report.

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