Mortars rained down Sunday on the courtyard of a girls' school in a mostly Sunni neighborhood of Baghdad, killing five pupils and wounding 20 others, police said. Iraqi forces backed by U.S. helicopters battled gunmen near Najaf to block an alleged plot to assassinate Shiite pilgrims and clerics during an upcoming religious festival.

The provincial governor said an American helicopter went down in the fighting near Najaf, but the U.S. military declined to comment on the report.

Elsewhere, two car bombs exploded within a half hour Sunday in the northern oil city of Kirkuk, killing a total of 11 people and wounding 34, police Brig. Gen. Sarhad Qader said. Three ethnic groups — Arabs, Kurds and Turkomen — are locked in a bitter struggle for control of the vast oil riches in the Kirkuk area.

Complete coverage is available in FOXNews.com's Iraq Center.

Also Sunday, the U.S. military reported the deaths of three more American service members — one Marine in the Sunni insurgent stronghold of Anbar province and two Army soldiers in the Baghdad area. All three died Saturday, but their names were withheld until their families are notified, the military said.

The mortar attack occurred about 11 a.m. at the Kholoud Secondary School in the Adil neighborhood of western Baghdad, police and school officials said. The principal, Fawzyaa Hatrosh Sawadi, said the students were mingling in the courtyard during a break in exams when at least two mortars exploded there.

The blasts shattered windows in the classrooms, spraying students with shards of glass. Associated Press Television News footage showed pools of blood smeared on the stone steps and walkways. Hours after the attack, grieving parents wept as the bodies of their children were placed inside wooden coffins.

Police said four girls were killed instantly and a fifth died later. AP Television footage showed the fin from one of the mortars lying on a blood-smeared walkway.

No group claimed responsibility for the attack, but a Sunni organization, the General Conference of the People of Iraq, accused Shiite militias with links to government security forces. The organization said in a statement that the mortars bore markings indicating they were manufactured in Iran, which U.S. officials accuse of supporting Shiite militias.

To the south, fierce fighting broke out Sunday when Iraqi troops backed by U.S. helicopters launched an attack against up to 200 insurgents in the Zaraq area about 12 miles northeast of the Shiite holy city of Najaf, provincial Gov. Assad Sultan Abu Kilel said. The provincial leader said an American helicopter went down during the battle.

The governor told reporters the insurgents were from the previously unknown "Army of Heaven" and were planning to kill Shiite pilgrims and clerics during ceremonies marking Ashoura, the holiest day in the Shiite calendar. Ashoura, which commemorates the death of a 7th century Shiite saint, culminates Tuesday in massive public processions in Karbala and other Shiite cities.

Government troops launched the attack at dawn but the gunmen, who were hiding in orchards, fought back with automatic weapons, sniper riles and rockets, the governor said. Two policemen were killed and 15 wounded along with an undetermined number of gunmen, he said.

Abu Kilel said the government was sending in reinforcements to finish off the militants and prevent them from escaping.

"They are well-equipped and they even have anti-aircraft missiles," Abu Kilel said. "They are backed by some locals" loyal to ousted dictator Saddam Hussein.

The two car bombs in Kirkuk exploded within a half hour in different parts of the city, 180 miles north of Baghdad. The first blast occurred at a car dealership, killing six people and wounding 19, police Brig. Gen. Sarhad Qader said. The second went off at a popular restaurant, killing five and injuring 15, Qader said.

In Baghdad, police said they found 39 bullet-riddled bodies throughout the city Sunday, apparent victims of sectarian militias. Another 10 bodies were recovered floating down the Tigris River 25 miles south of the capital.

A series of smallscale bombings also struck Shiite parts of the capital. One person was killed and five were wounded when a bomb exploded in a minibus carrying passengers to the predominantly Shiite neighborhood of Sadr City, police said.

Five hours later, a car bomb exploded near an outdoor market in Sadr City, killing at least four people, two of them women, and wounding 39, police said. The sprawling Shiite slum is a stronghold of the Mahdi Army of radical cleric Muqtada al-Sadr, which has blamed for much of the sectarian violence.

At about the same time, a bomb hidden in a bag exploded in an outdoor market in the Baiyaa neighborhood in western Baghdad, an area that is mostly Shiite. At least two people were killed and 17 were wounded, including two children, police said.

Also Sunday, drive-by shooters killed a high-ranking Shiite official at the Industry and Mines Ministry, along with his 27-year-old daughter and two other people, police said. A car bomb also exploded Sunday near a mosque in the Sunni city of Fallujah, 40 miles west of Baghdad, killing two civilians and wounding four, police said.

The blasts and the attack on the girls' school appeared to be part of the pattern of sectarian reprisal killings that have pushed the Iraq into civil warfare. President Bush has announced plans to send up to 21,500 troops to Baghdad and surrounding areas in a bid to stem the violence.

U.S. officials have long blamed Al Qaeda in Iraq for fanning sectarian hatreds by vicious attacks against Shiite civilians. Revenge killings surged after the Feb. 22 bombing of a Shiite shrine in the largely Sunni city of Samarra.

On Sunday, the U.S. command announced the arrest of 21 suspected terrorists including an Al Qaeda courier in a series of raids in Baghdad and Sunni areas north and west of the capital. Three of the suspects were believed to have close ties to the leadership of Al Qaeda in Iraq, the military said.

Complete coverage is available in FOXNews.com's Iraq Center.