A group of nine men wearing Iraqi police uniforms stormed an elementary school south of Baghdad Monday, ordered all the male teachers and the school bus driver into a room and sprayed them with bullets, killing six, in a rare attack on a school in Iraq, officials said.

"These men were terrorists wearing police uniforms," said Capt. Muthana Ahmed, director of the provincial police force in the area about the attackers. "No prior warning was made."

Ahmed said the children at the Jazeerah Primary School in a village near Iskandariyah were not in the room where the teachers and the school bus driver, all Shiites, were shot.

A suicide bomber also killed at least 10 people outside Iraq's oil ministry Monday when he rammed his car into a bus carrying police and oil ministry officials. Seven policemen and three members of the country's oil ministry were killed, police said.

Three U.S. soldiers also were killed in two separate attacks in the southeast and west of Baghdad, according to U.S. military and wire service reports. All three died when their vehicles were struck by roadside bombs. No further details were reported.

Sectarian killings have been rife in Iraq, where the Shiite and Kurdish-dominated government, backed by U.S. forces, is facing an armed insurgency by Sunni Arabs. Attacks on Shiites, who make up the vast majority of the population, have increased in the country in the run-up to the Oct. 15 referendum on the country's new constitution. Most Sunnis, who lost political control after Saddam Hussein was ousted as president in 2003, have voiced opposition to the document. Sunnis make up about 20 percent of the population.

The two attacks Monday followed a deadly Sunday in Iraq.

A suicide bomber driving a minibus pulled alongside a convoy of elite Iraqi commandos in the capital Sunday and detonated his explosives, killing 10 people in a spray of burning metal, witnesses said.

The powerful explosion rattled windows miles from the stretch of highway in the eastern neighborhood of Ghadeer. Blood from the dead and injured soaked bread in roadside stalls.

Hours later, Ahmed Dawood, 25, a trader who was driving on the highway at the time and witnessed the explosion, was still shuffling around in shock, carefully stepping over sandals and pieces of flesh.

"The highway was so crowded," said Dawood, dazed but uninjured. "When the explosion happened, I searched for a place to hide because there was heavy shooting."

Iraqi officials provided no immediate count of the casualties. Witnesses said they saw at least 10 charred bodies, the majority of whom were commandos, part of a highly trained Interior Ministry protection force. Unlike U.S. forces in Iraq, who travel in armored vehicles, the commandos were riding in the back of a pickup truck.

Six people were also killed Sunday morning in Sadr City, a large Shiite slum in Baghdad, when U.S. forces and Iraqi commandos exchanged gunfire with members of the Mahdi Army, a militia loyal to the popular Shiite cleric, Moqtada Sadr.

The Mahdi Army controls Sadr City, which has become one of the safest areas of the capital in recent months. Residents of the slum stroll the streets after dark, taking in the cooler night air when the power goes out -- an activity unheard of in other parts of Baghdad.

The U.S. military said it had no immediate information on the incident.

Maj. Gen. Raad Tamimi, an Interior Ministry official, said the clashes began when an armed group of Sadr's followers gathered near a mosque, raising the suspicions of U.S. and Iraqi troops on patrol there. The U.S. forces "ordered the armed men to withdraw, but they refused and moved toward them," prompting the Americans to shoot, Tamimi said.

But residents of Sadr City said that the Iraqi troops had joined the Americans in an unprovoked assault on the Mahdi Army members.

Waleed Wadi Lafta, a resident who said he witnessed the exchange of gunfire, said "there were six pickups of the Interior Ministry commandos" and six American Humvees. "They came to arrest a man whom they did not find."

The U.S. and Iraqi forces then "started shooting randomly at the people who were in the street," he said. "They shot at the mosque and the nearby shops."

The gate of the mosque was riddled with bullet holes, but it was not possible to independently confirm how or when it was damaged. The tall, spiraling minaret of the mosque was also pocked with bullet holes.

Crowds gathered in front of the mosque cursed what they called the "occupation" and the government.

"If the government cares about us, they should not send others to kill us," said Mohammed Sabah, 23, a high school teacher and resident of Sadr City.

In the Shiite holy city of Najaf, 90 miles south of Baghdad, Sayyid Riyadh Nouri, a senior aide to Sadr, said that the cleric had asked people not to retaliate. "Our leader, Sayyid Moqtada Sadr, ordered the people to be in the highest self-control and not to be engaged in any military fighting," he said.

Nouri called for immediate government intervention "to stop this crisis that will lead to the instability in the country."

Meanwhile, a U.S. soldier was killed Sunday when his vehicle rolled over during a patrol in western Iraq, the military said. Two other soldiers were injured in the accident in Trebil, near the border with Jordan, according to the Associated Press. No further details were available.

In Hilla, 55 miles south of Baghdad, five Iraqi civilians were killed and 49 were injured when a booby-trapped bicycle exploded in front of an ice-cream parlor and juice bar in the center of the city, said Capt. Muthanna Ahmed, a press officer at the Babil province police directorate. He said many of the injured were in critical condition.

At Hilla's General Hospital, Hadi Alwan, 36, who was among the injured, said the area was crowded with people when the explosion happened. "I did not remember anything, except my legs were bleeding and then I fainted," he said.

Maj. Gen. Qais Hamza, police chief for the province, ordered the hospital to prevent injured patients from leaving because police suspected that the bomber might be among them.

In Samarra, about 65 miles north of Baghdad, armed men shot and killed five Iraqi security guards employed by Northern Oil Co., police Lt. Raed Mahdi said. In the same area, the bodies of three unidentified people who had been shot were found in the Tigris River, Mahdi said.

Special correspondents Saad Sarhan in Hilla and Salih Saif Aldin in Samarra contributed to this report.