With the revered Shiite city of Karbala and the strategic city of Kut behind them, U.S. troops have entered the "red zone," the swath of territory at Baghdad's doorstep and the region feared for the potential of chemical attack.

Speculation about the "red zone" has become ominous and almost mythical as coalition forces advance on the Iraqi capital and near the point where Saddam Hussein's forces might mount a no-holds-barred last stand.

On the ground, it has had very real implications: troops have been in charcoal-lined protective suits for much of the past two weeks, gas masks at the ready.

Having breached the zone Wednesday, two of the commands closest to Baghdad -- a 3rd Division infantry brigade and the Marines who pounded the Republican Guard's Baghdad Division west of Kut -- went on heightened chemical alert.

"Our forces are now on extra alert for potential use for chemical or other destructive weapons by the regime," said Jim Wilkinson, spokesman at U.S. Central Command.

Iraq denies still having chemical or biological weapons, which were ordered destroyed under U.N. resolutions that ended the 1991 Gulf War. The United States and its allies launched Operation Iraqi Freedom last month, claiming Baghdad still has weapons of mass destruction and was prepared to use them.

Brig. Gen. Vincent Brooks, Central Command deputy operations director, described the red line as a "conceptual line" -- not necessarily a physical one. The fear is that once the line is crossed it might trigger Saddam's forces to use of chemical weapons in desperation.

That point may come as coalition troops press further toward Baghdad itself and squeeze withdrawing Iraqi troops at the gates of the city.

"This is what we conceive could be a trigger point, and that's based on the radius of weapon systems that might be used, where we think the threat to the regime might be viewed as the greatest," Brooks told reporters.

Iraq is believed to have mortar shells, artillery and short-range missiles capable of carrying chemical weapons, including the FROG-7, which has a 40-mile range. Iraq used FROG-7s with mustard gas warheads during the 1980-88 Iran-Iraq War, according to Jane's Defense Weekly.

A military official here said the imaginary red line, the conceptual trip wire for the danger zone, runs east from Karbala, about 50 miles south of Baghdad on the Euphrates River, to Kut on the Tigris River southeast of Baghdad.

Members of 5th Corps, the 3rd Infantry Division and the 82nd Airborne conquered the Republican Guards' Medina divisions at Karbala on Wednesday while members of the 1st Marine Expeditionary Force destroyed the Baghdad Division around Kut -- and both pressed on toward Baghdad.

Brooks said coalition airstrikes and other attacks have concentrated so much on Iraqi command and control facilities in a bid to prevent issuance of orders to use chemical arms.

"It's all about preventing the action as much as possible," he said.

"And if we're successful in that, that's very good. That's the desired outcome. We don't want to see weapons of mass destruction used. Just like our leaflets say, no one benefits from the use of weapons of mass destruction."

"The rest of the story is known only to the regime, and we will not ever know that," he said. "If we're successful, they will never be used, and this red line will have been something that we just conceived and it was not real. And that's fine."

But he stressed: "If it is used, we'll be prepared for it to be used."