U.S. troops near Baghdad, attack Republican Guard outside city.

Byline: S. Thorne Harper, Juan O. Tamayo and Martin Merzer

NEAR KARBALA, Iraq _ The opening phase of the battle for Baghdad erupted early Wednesday as thousands of U.S. Army troops and Marines thundered into action, approaching Republican Guard divisions that block the southern passages to the capital.

In the full-scale ground attack, all three brigades of the Army's 3rd Infantry Division drove toward a Republican Guard division southwest of Baghdad while a large force of Marines lunged for a Republican Guard division southeast of the capital.

"We continue to tighten the noose around Baghdad," said Lt. Col. George Smith at Marine combat headquarters in Iraq.

Only light mortar fire and other minor resistance was reported in the early hours of the attack and no immediate reports surfaced of U.S. casualties. On Tuesday, one U.S. soldier was killed in south-central Iraq, raising the U.S. death toll to 49 since the war began.

The largest U.S. military assault since the Persian Gulf War in 1991, the invasion moved along two fronts and marked the beginning of what was expected to be a key battle of this second Gulf War _ the struggle to breach Republican Guard lines and reach the seat of Saddam Hussein's power about 50 miles away.

If successful, the double-barreled advance _ which followed a four-day halt in large-scale movement _ could strip away the outer layer of Baghdad's defenses and leave only two other Guard divisions standing between U.S. forces and Saddam.