Associated Press military writer

WASHINGTON – U.S. Army forces killed between 150 and 500 Iraqi troops Tuesday in a fierce fight after coming under attack near the central Iraqi city of An Najaf, a senior defense official said.

No U.S. casualties were reported, although the official cautioned that few details were immediately available.

Elements of the 7th Cavalry Regiment were east of An Najaf when they suddenly came under fire from rocket-propelled grendes, the official said. The Iraqis were on foot and it was not clear whether they were from regular army units, paramilitary forces or the Republican Guard.

Some of the 7th Cavalry’s equipment was damaged in the attack, the official said.

Early estimates of the number of Iraqis killed in the fight varied widely, from 150 to 500. The Americans fired mainly the 25mm guns on their Bradley fighting vehicles, according to another U.S. official.

The U.S. troops were hunkered down during a severe sandstorm when they were attacked by a combination of Republican Guard and paramilitary Iraqi troops, another U.S. official said.

The attacks suggested the location of Iraqi strongholds in the area, and the U.S. troops used thermal-imaging equipment to kill a large number of Iraqis as the sandstorm raged, the second official said.

The 7th Cavalry is part of the Army force driving toward Baghdad. Some elements of the force are farther north, near Karbala, with only the Medina armored division of the Republican Guard between them and Baghdad.

U.S. intelligence, meanwhile, has picked up signs suggesting the closer ground troops get to Baghdad the greater the chance they will face chemical weapons, Defense Secretary Donald H. Rumsfeld said Tuesday.

The Army’s 3rd Infantry Division has drawn to within about 50 miles of Baghdad, with the Medina armored division of Saddam Hussein’s Republican Guard in its path. Elements of the 1st Marine Division are approaching the capital from a more easterly direction, and some analysts believe the Army’s 101st Airborne Division, now in southern Iraq, will join the battle for Baghdad.

Asked about reports that Republican Guard forces ringing Baghdad have been given authority to use chemical weapons, Rumsfeld cited &uot;scraps” of intelligence suggesting that the closer the 3rd Infantry gets to the capital, the greater the danger.

He did not offer details of the intelligence indicators because &uot;who knows how accurate they are,” he said.

Iraq denies it has any chemical or biological weapons. The Bush administration insists it has both and is trying to gain nuclear weapons. The risk of Iraq using chemical, biological or nuclear weapons or providing them to terrorist networks was the central reason President Bush went to war.

Gen. Richard Myers, chairman of the Joint Chiefs of Staff, said the commander of U.S. forces fighting the war, Gen. Tommy Franks, has plans in place should Iraq use chemical weapons. He would not elaborate.

Secretary of State Colin Powell, in an interview with France 3 television, cited speculation that &uot;there is a box around Baghdad, that if we penetrate that box,” Saddam would unleash a chemical attack. &uot;If he did,” Powell added, &uot;it would not stop the (U.S.) assault.”

U.S. forces are equipped with full-body chemical protection suits and gas masks.

At a joint Pentagon news conference with Myers, Rumsfeld tried to dampen public expectations that the war would be won quickly and to reiterate the message delivered daily by senior military commanders here and in the Persian Gulf that the war is progressing as planned.

&uot;We’re still, needless to say, much closer to the beginning than the end,” Rumsfeld said.

Rumsfeld brushed aside suggestions that U.S. war planners underestimated the Iraqis’ will to fight and overestimated the ability of U.S. precision airstrikes against the pillars of Saddam’s power to end the conflict before it reached the stage of having to initiate a battle for Baghdad.

Some private analysts, including former military officers, have suggested that the Army needs more armored forces in Iraq than the 20,000 troops of the 3rd Infantry Division, including forces that could better protect the 3rd Infantry’s long and vulnerable supply lines.

&uot;Forces increase in the country every minute and every hour of every day, and that will continue to be the case,” Rumsfeld said. He apparently referred to the Army’s 4th Infantry Division, which is expected to deploy from its Fort Hood, Texas, home base toward the end of this week.

Also headed for Kuwait soon is the 3rd Armored Cavalry Regiment from Fort Carson, Colo., followed by the 1st Infantry and 1st Armored divisions from Germany, as well as the 1st Cavalry Division from Fort Hood, Texas, and the 2nd Armored Cavalry Regiment from Fort Polk, La., officials said.

The 4th Infantry had been designated to deploy to Turkey in February to open a northern front against Iraq, but the Turkish government refused to grant access. So Franks has ordered the division’s weaponry and equipment shipped to Kuwait, where it is due to arrive in about two weeks.