Baghdad, Iraq ? Iraqi and American forces killed several hundred fighters apparently trying to ambush religious pilgrims Sunday in the holy city of Najaf, during a daylong battle in which a U.S. helicopter crashed, killing two U.S. troops, Iraqi security officials said.

The fighting, on the eve of the Shiite Muslim holiday of Ashura, came as a mortar attack killed five teenage girls at a school in Baghdad and the daily civilian death toll nationwide again climbed past 100.

Iraqi security officials offered conflicting accounts of the identity and motives of the heavily armed fighters in Najaf, variously describing them as foreign fighters, Sunni Arab nationalists, loyalists of executed former dictator Saddam Hussein or followers of a messianic Shiite death cult. Some witness reported the attackers wore colorful Afghan tribal robes.

The cause of the helicopter crash in Najaf was unclear, but U.S. and Iraqi officials said it was felled by fire from the ground, and witnesses said they saw it shot out of the sky. It was the third helicopter to go down in eight days.

Three additional U.S. troops were reported killed Sunday, and at least 113 Iraqis were killed or found dead.

Sunday’s fighting in Najaf and elsewhere was extraordinary, even by Iraq’s bloody standards, highlighting the daunting challenge faced by U.S. and Iraqi forces, which are fighting a complex patchwork of elusive enemies, including Shiite militias and Sunni Arab insurgents.

Iraqi security forces took authority over Najaf’s security about a month ago. But witnesses and security officials said Sunday that Iraqi forces were being beaten by the enigmatic but well-organized fighters until U.S. forces and air support arrived.

Shaky footage recorded by mobile telephone, broadcast on Iraqi television, showed Iraqi soldiers hunkered behind a berm as intense gunfire erupted and smoke rose in the distance.

End of days

Ali Nomas, an Iraqi security official in Najaf, said the fighters belonged to a group calling itself Heaven’s Army – one among several messianic cults that have appeared among Shiites, who believe in the imminent return of Imam Mahdi, the last in the line of Shiite saints who disappeared more than 1,000 years ago. Nomas said the information came from interviews with at least 10 detained fighters.

“Every day someone claims he’s the Mahdi,” he said.

Nomas said the leader of the hitherto unknown Heaven’s Army had told followers that he was a missing son of the Imam Ali, the cousin and son-in-law of the Prophet Muhammad. Ali’s remains are entombed in Najaf.

“They believe that the Mahdi has called them to fight in Najaf,” he said, adding that fighters had converged on Najaf from other predominantly Shiite cities in Iraq.

Nomas lamented that Iraq’s death and destruction had convinced some Shiites that the end of days was coming.

“There’s nothing bizarre left in Iraq any more,” he said in a telephone interview. “We’ve seen the most incredible things.”

But Najaf Gov. Asaad Abu Gulal said some of the fighters were members of Saddam’s Baath Party, and said the group planned to attack pilgrims and shrines and to assassinate Shiite clerics at the peak of the holiday.

“Today’s attack was designed to destroy all of Najaf, even the holy shrine of Imam Ali,” said Ahmed Duaibel, a spokesman for the provincial government in Najaf. If successful, such a provocative attack could surpass in significance the bombing at the Askariya shrine in Samarra last February that drastically escalated sectarian killing in Iraq.

U.S. reinforcement

Although they disagreed on the attackers’ identity, Iraqi officials and witnesses offered the strikingly similar account of events on the battlefield. Most of the fighting took place in the farmland outside the city, which also is home to the most revered Shiite cleric, Grand Ayatollah Ali Sistani. Security forces cordoned off the ancient labyrinthine city to prevent attacks on pilgrims, clergy and holy sites, the governor said.

The gunmen, numbering at least 500, planned to launch their attack today, on the 10th day of the Muslim lunar month of Muharram, the most holy day in the Shiite calendar. But Iraqi security forces were tipped off Sunday night about their presence on nearby farms, Gulal said.

Iraqi security forces struck at dawn but were overwhelmed by the militants, who had dug trenches on farms. At least two Iraqi soldiers were killed in the initial fighting, a security official in Baghdad said.

Iraqi forces then called in U.S. air support as well as the Scorpion Brigades, an Iraqi quick-reaction force based in a neighboring province.

Helicopters arrived, but after one crashed at around 1:30 p.m., they were replaced by higher-flying jets, as American Humvees and armored vehicles rolled into the area.

Three more Iraqi soldiers were killed, as were at least 250 of the fighters, according to several Iraqi officials. Those numbers could not be independently confirmed. By 4 p.m., the tide of the battle had shifted, but U.S. forces continued bombing into the night in an attempt to stamp out remnants of the militants, Iraqi officials and witnesses said.