BAGHDAD, IRAQ - A mysterious group of religious zealots who fought a fierce battle with American and Iraqi troops on Sunday had AK-47 rifles, rocket-propelled grenades and five anti-aircraft machine guns on tractors -- raising questions about how a group with no known ties to Iraq's political organizations was so well equipped and trained.

"They fought according to a military arrangement, and they moved as platoons and companies," Abdul Hussein Abtan, the deputy governor of Najaf, said Monday.

The Iraqi army was surprised and nearly overwhelmed by the ferocity of the militia and needed far more help from American forces than previously disclosed, American and Iraqi officials told The New York Times.

They said American ground troops -- and not just air support as reported Sunday -- were mobilized to help the Iraqi soldiers, who appeared to have dangerously underestimated the strength of the militia, which calls itself the "Soldiers of Heaven" and had amassed hundreds of heavily armed fighters.

Iraqi government officials said the group apparently was preparing to storm Najaf, a holy city dear to Shiite Islam, occupy the sacred Imam Ali mosque and assassinate the religious hierarchy there, including the revered leader, Ayatollah Ali al-Sistani, during the height of Ashura, an important Shiite holiday when large numbers of pilgrims visit.

"This group had more capabilities than the government," said Abdul Hussein Abtan, the deputy governor of Najaf Province, at a news conference.

Ali Nomas, a spokesman for the security forces in Najaf, said the militants, who numbered from 1,000 to 1,500, had purchased farms and surrounded them with a dirt barricade and a bulldozed trench. More than 2,000 AK-47 assault rifles and 700 rocket-propelled grenades were recovered after the battle, Nomas said.

Among the 300 or so militants killed in Sunday's fighting was the group's leader, Iraqi authorities said. As many as 400 others were arrested, including some dressed as Afghan fighters, Iraqi spokesmen said. U.S. officials put the number of arrests at more than 100.

A U.S. helicopter was shot down during the fighting. Both crewmen died.

Maj. Hussain Muhammed of the Iraqi army said some fighters escaped.

"We have information that a large number of fighters have escaped through the palm groves. Some were wearing the uniforms of the security forces and others were wearing black," Muhammed said.

Even in Iraq's volatile and violent brew of sectarian, political, tribal and ethnic factionalism, the explosive emergence of the "Soldiers of Heaven" stands apart as a reminder of how little understanding there is of the country's complex web of militias.

The battle also brought into focus the reality that some of the power struggles in Iraq are among Shiites, not just between Shiites and Sunnis. The "Soldiers of Heaven" is considered to be at least partly or wholly Shiite-run.

Iraqis have been speculating wildly and contradictorily, asserting that they recognize elements of Shiite, Sunni and other influences among the militants.

Asad abu Kalal, the governor of Najaf, said as much himself in a press conference on Monday.

"In external form, the way they look is Shiite, but its reality is something else," Kalal said. "They meant to destroy the Shiite and kill the Grand Marjiyas and occupy the Holy Shrine of Imam Ali," he said. The Grand Marjiyas are the four leading ayatollahs in Najaf. They are led by Grand Ayatollah Ali al Sistani, Iraq's top Shiite cleric.

Among other troubling questions raised about the Iraqis' military ability is how hundreds of armed men were able to set up such an elaborate encampment, which Iraqi officials said included tunnels, trenches and a series of blockades, only 10 miles northeast of Najaf. After the fight was over, Iraqi officials said they discovered at least two anti-aircraft weapons as well as 40 heavy machine guns.

The government knew that the "Soldiers of Heaven" had set up camp in the area, but officials thought they were there to worship together.

Last month, U.S. forces turned over security for the Najaf region to provincial authorities. According to those authorities, the "Soldiers of Heaven" already were arming themselves on a series of farms they'd recently bought.

Abtan, Najaf's deputy governor, said only a few fighters lived on the farms and that they worked to build its battlements and caches. The others were summoned in the past few days.

A U.S. military statement said that Iraqi security forces received a tip about the militants and moved out of Najaf on Sunday morning to confront them.

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