The US Army's 3rd Infantry Division (3ID) was spearheading the attack after moving through a strategic corridor west of the city of Karbala while US marines moved on Baghdad from the southeast, crossing the Tigris River near the city of Kut.

Asked how close were the lead elements of the attacking force, Major Maurice Goins, operations officer of the 3ID's 1st brigade, said: "I'd say within 50 miles of Baghdad."

A senior US commander said Wednesday that troops pushing closer to Baghdad have been put on high alert against chemical or biological attack amid fears Iraqi President Saddam Hussein may have drawn a "trigger line" in the sand.

"There may be a trigger line where the regime sees a sufficient threat to use weapons of mass destruction," Brigadier General Vincent Brooks told reporters at the Central Command forward headquarters in Qatar.

"That's the red line. It's a conceptual line across which there may be a decision made. That's why we attack the regime's methods of communications, that's why we attack those who would make decisions.

"It's about preventing the action as much as possible."

Officers ordered soldiers to move up to the next level of precaution against chemical or biological weapon attack by donning rubber boots along with the protective clothing they have worn almost every day since the US-British invasion began on March 21.

Brooks last week warned that intelligence officers had received "indications through a variety of sources" that the Iraqi regime had issued "first orders" to use chemical or biological weapons if the coalition forces crossed the line.

"We have seen chemical protective equipment in a number of areas south of where we thought that red line might be," he said last week, adding however that he had not seen "anything saying that an order has been given to fire".

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SPACE.WIRE