Major General Stanley McChrystal, vice director of operations of the Joint Staff, said US forces were bracing for tougher fighting ahead and did not expect to seize Baghdad in a "coup de main."

But he said the fighting "is clearly threatening Baghdad and threatening the core of the regime."

The Medina and Baghdad Republican Guard divisions, which were defending the approaches to Baghdad at Karbalba and Al Kut respectively, were effectively destroyed in the fighting, he said.

McChrystal said that meant they "cannot maneuver as a division, cannot defend effectively, is not effectively able to counterattack. And that's what we're seeing with a couple of these divisions."

"We are seeing some surrender, but not in tremendous numbers. We are essentially able to move through. It's unclear what is happening to some of those elements," he said.

The general also discounted the possibility that the Republican Guard formations were retreating into the city.

"As we are destroying many of those organizations in place, their equipment in place, as people melt away, it's very difficult to stop very small groups of people. But organized formations would have a very difficult time pulling out," he said.

Remnants of those forces were putting up sporadic resistance, however, and other Republican Guard units and regular army troops have been brought in to reinforce the sectors they were defending, he said.

McChrystal said the Iraqi military still exhibited some control over their forces since they were maneuvering.

"But having said that, there is command and control, but effective command and control, effective maneuvers is not as evident," he said.

Pentagon spokeswoman Victoria Clarke, who has sought throughout the conflict to keep expectations in check, told reporters "we want to underscore again that some of the toughest fighting may well lie ahead."

"They're fighting. We have superior forces, superior capabilities. The end is inevitable. We know how this is going to end. But they are fighting. They're not just sitting there waiting for this to happen to them," she said.

Concurring, McChrystal said, "We are not expecting to drive into Baghdad suddenly and seize it in a coup de main or anything like that."

He said potential use of chemical and biological weapons remained a concern although no such use has been reported so far in the 13-day old ground war.

"Clearly, as we threaten the core of the regime which Baghdad and Tikrit represent, we believe that the likelihood of them using those weapons goes up. And so, the posture of our force is prepared for that," he said.

He warned "it'll be a grave mistake for either who orders it or the people who execute it; but it also won't change the outcome on the ground. We'll still be able to maneuver, we'll still be able to execute the operations as planned."

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