Mortar shells, meanwhile, hit the courtyard of a girls' school in a mostly Sunni Arab neighborhood of Baghdad, killing five pupils and wounding 20. U.N. officials deplored the attack, calling the apparent targeting of children "an unforgivable crime."

Two car bombs exploded within a half hour in the northern city of Kirkuk, killing 11 people and wounding 34, police Brig. Gen. Sarhad Qader said. Three ethnic groups - Arabs, Kurds and Turkomen - are in a bitter struggle for control of that oil-rich area.

In addition to confirming the two Americans killed in the helicopter crash near Najaf, the US command announced three combat deaths from Saturday - one Marine in the Sunni insurgent stronghold of Anbar province and two Army soldiers in the Baghdad area.

Authorities said Iraqi soldiers supported by US aircraft fought all day with a large group of insurgents in the Zaraq area, about 12 miles northeast of the Shiite holy city of Najaf.

Col. Ali Nomas, spokesman for Iraqi security forces in Najaf, said more than 250 corpses had been found. Iraqi army Maj. Gen. Othman al-Ghanemi also spoke of 250 dead but said an exact number would not be released until Monday. He said 10 gunmen had been captured, including one Sudanese.

Provincial Gov. Assad Sultan Abu Kilel said the assault was launched because the insurgents planned to attack Shiite pilgrims and clerics during ceremonies marking Ashoura, the holiest day in the Shiite calendar commemorating the 7th century death of Imam Hussein. The celebration culminates Tuesday in huge public processions in Karbala and other Shiite cities.

Officials were unclear about the religious affiliation of the militants. Although Sunni Arabs have been the main force behind insurgent groups, there are a number of Shiite militant and splinter groups that have clashed from time to time with the government.

Iraqi soldiers attacked at dawn and militants hiding in orchards fought back with automatic weapons, sniper rifles and rockets, the governor said. He said the insurgents were members of a previously unknown group called the Army of Heaven.

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

Abu Kilel said two Iraqi policemen were killed and 15 wounded, but there was no word on other Iraqi government casualties.

A US statement said the American helicopter went down while "conducting operations to assist Iraqi Security Forces" in the attack. It said two crew members died and their bodies were recovered. The statement did not give any information on why the aircraft crashed.

It was the second US military helicopter to do down in eight days. Twelve US soldiers died Jan. 20 when a Black Hawk crashed northeast of Baghdad. The Army says it is investigating the cause, but a Pentagon official has said debris indicated it was downed by a missile.

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

The blasts shattered windows in classrooms, spraying students with shards of glass. Associated Press Television News footage showed pools of blood on the stone steps and walkways. A fin from a mortar shell lay on the ground.

Hours after the attack, grieving parents wept as the bodies of their children were placed in wooden coffins. Police said four of the girls were killed instantly and a fifth died later.

In a joint statement, UNICEF and UNESCO called the attack "yet another tragic reminder of the risks facing Iraq's schoolchildren."

No group claimed responsibility for the attack, but a Sunni organization, the General Conference of the People of Iraq, blamed Shiite Muslim militias with ties to government security forces. The group said in a statement that the mortar shells bore markings indicating they were manufactured in Iran, which US officials accuse of supporting Shiite militias.

Three bombings, meanwhile, struck Shiite districts in Baghdad, killing at least seven people and wounding 61, police said.

The worst incident was a car bomb that killed at least four and wounded 39 at an outdoor market in Sadr City, a sprawling slum that is a stronghold of the Mahdi Army of radical cleric Muqtada al-Sadr, a militia blamed for much of Iraq's sectarian bloodshed.

The mortar attack and bombings appeared to be part of the sectarian reprisal killings that have pushed Iraq into civil warfare over the past year, violence that President Bush hopes to quell by sending up to 21,500 more American soldiers to Baghdad and surrounding areas.

US officials have long accused al-Qaida in Iraq, a Sunni Muslim group, of fanning sectarian hatreds by staging vicious attacks on Shiite civilians. Revenge killings have surged since the bombing of a Shiite shrine in the largely Sunni city of Samarra last Feb. 22.

The two car bombs in Kirkuk exploded within 30 minutes of each other in different parts of the city, 180 miles north of Baghdad. The first blast was at a car dealership, killing six people and wounding 19, said Qader, the police general said. The second went off at a popular restaurant, killing five and injuring 15, he said.

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

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 exploded near a mosque in the Sunni city of Fallujah, 40 miles west of Baghdad, killing two civilians and wounding four, police said.

The US command announced the arrest of 21 suspected terrorists, including an al-Qaida courier, in a series of raids in Baghdad and Sunni areas north and west of the capital. Three are believed to have close ties to the leadership of al-Qaida in Iraq, the military said.