An attack by hundreds of Iraqi militia members on the U.S. government's headquarters in Najaf on Sunday was repulsed not by the U.S. military, but by eight commandos from a private security firm, according to sources familiar with the incident.

Before U.S. reinforcements could arrive, the firm, Blackwater Security Consulting, sent in its own helicopters amid an intense firefight to resupply its commandos with ammunition and to ferry out a wounded Marine, the sources said.

The role of Blackwater's commandos in Sunday's fighting in Najaf illuminates the gray zone between their formal role as bodyguards and the realities of operating in an active war zone. Thousands of armed private security contractors are operating in Iraq in a wide variety of missions and exchanging fire with Iraqis every day, according to informal after-action reports from several companies.

In Sunday's fighting, Shiite militia forces barraged the Blackwater commandos, four MPs and a Marine gunner with rocket-propelled grenades and AK-47 fire for hours before U.S. Special Forces troops arrived. A sniper on a nearby roof apparently wounded three men. U.S. troops faced heavy fighting in several Iraqi cities that day.

The Blackwater commandos, most of whom are former Special Forces troops, are on contract to provide security for the U.S.-led Coalition Provisional Authority (CPA) in Najaf.

With their ammunition nearly gone, a wounded and badly bleeding Marine on the rooftop, and no reinforcement by the U.S. military in the immediate offing, the company sent in helicopters to drop ammunition and pick up the Marine.

The identity of the Marine and two other wounded men could not be established, but their blood was still fresh hours later, when the top U.S. commander in Iraq, Lt. Gen. Ricardo Sanchez, and spokesman Brig. Gen. Mark Kimmitt arrived to survey the battle zone.

Without commenting at a news conference yesterday on the role of the Blackwater guards, Kimmitt described what he saw after the fighting ended. "I know on a rooftop yesterday in An Najaf, with a small group of American soldiers and coalition soldiers . . . who had just been through about 3 1/2 hours of combat, I looked in their eyes, there was no crisis.

"They knew what they were here for," he continued. "They'd lost three wounded. We were sitting there among the bullet shells -- the bullet casings -- and, frankly, the blood of their comrades, and they were absolutely confident."

During the defense of the authority headquarters, thousands of rounds were fired and hundreds of 40mm grenades shot. Sources who asked not to be identified because of the sensitivity of Blackwater's work in Iraq reported an unspecified number of casualties among Iraqis.

A spokesman for Blackwater confirmed that the company has a contract to provide security to the CPA but would not describe the incident that unfolded Sunday.

A Defense Department spokesman said that there were no military reports about the opening hours of the siege on CPA headquarters in Najaf because there were no military personnel on the scene. The Defense Department often does not have a clear handle on the daily actions of security contractors because the contractors work directly for the coalition authority, which coordinates and communicates on a limited basis through the normal military chain of command.

The four men brutally slain Wednesday in Fallujah were also Blackwater employees and were operating in the Sunni triangle area under more hazardous conditions -- unarmored cars with no apparent backup -- than the U.S. military or the CIA permit.

One senior Blackwater manager has described those killings to U.S. government officials as the result of a "high-quality" attack as skilled as one that can be mounted by U.S. Special Forces, according to a copy of a report on the incident obtained by The Washington Post.

The four victims of that attack, according to Blackwater spokesman Chris Bertelli, were escorting trucks carrying either food or kitchen equipment for Regency Hotel and Hospitality. Regency is a subcontractor to Eurest Support Services (ESS), a division of the Compass Group, the world's largest food service company.

ESS provides food services to more than a dozen U.S. military dining facilities in Iraq, according to news accounts.

Blackwater, a security and training company based in Moyock, N.C., prides itself on the high caliber of its personnel, many of whom are former U.S. Navy SEALs. It has 450 employees in Iraq, many of them providing security to CPA employees, including the U.S. administrator, L. Paul Bremer, and to VIPs visiting Iraq.

Blackwater has applied to occupy a former MIG air base near Baghdad as a counterterrorism training facility for Iraqi forces. The training range will mirror the 6,000-acre Moyock site, which is frequented by U.S. law enforcement and military personnel.

Blackwater commandos and a U.S. soldier defend the Coalition Provisional Authority headquarters in Najaf.