The new offensive targeted against at least two Republican Guard divisions -- the Medina and Baghdad -- south of the Iraqi capital, in what could be the beginning of the battle for Baghdad.

One Pentagon official said U.S. forces are engaging Iraqi Republican Guard troops in a major fight in Karbala.

The new offensive came after U.S.-led coalition forces in the Persian Gulf region began receiving a battle plan suggesting the focus of the ground war would soon shift to Baghdad, according to U.S. military officials.

Gen. Tommy Franks, chief of U.S. Central Command, will not have to consult with President Bush or Defense Secretary Donald Rumsfeld before moving on the Iraqi capital, officials told CNN.

Franks is not locked into a time frame and will seize the "tactical advantage" when the time is right, the officials said.