In the southwest, lead elements of the Army's 3rd Infantry Division pushed through a gap west of Karbala and captured a bridge across the Euphrates River at Mussayib, about 40 miles from the capital. They met little resistance, facing only sporadic blasts of small arms and rocket-propelled grenade fire. 

The bridge was rigged with explosives, which were defused by engineers to allow U.S. units across the river. The units at the front of the infantry push put on their chemical protective gear as they drew inexorably closer to the battle for Baghdad. 

Eventually, the U.S.-led forces intend to launch a synchronized attack on Baghdad with the infantry, the Marines and the Air Force, said Navy Capt. Frank Thorp, a U.S. Central Command spokesman. 

"Synchronicity is a very strong doctrine of the United States army," Thorp said. "To synchronize the different battles on the field provides that tactical surprise." 

Their efforts could be hampered by the weather, with temperatures expected to climb into the 90s over the next several days. 

The Marines seized the strategic southeast town of Kut and routed a Republican Guard division that was guarding the highway to Baghdad. "The Baghdad Division no longer exists, and the 1st Marine Expeditionary Force is moving on," Thorp said. 

A military statement read over Iraqi satellite television called that report a lie, saying the division remained ready to fight. 

Some 25 miles to the northwest, in the same region where troops seized that key bridge, marine engineers were constructing a new one - a pontoon span across the Tigris near Numaniyah, which would provide them with a secure crossing to bring in troops and equipment, Associated Press Broadcast News reporter Ross Simpson said. 

At a canal near the Tigris, Marines under machine-gun fire had battled with Iraqi forces in heavy fighting to gain control of another bridge. Iraqi artillery shells and mortars exploded, while blasts of machine-gun fire tore apart buildings. U.S. forces estimated they had killed 100 Iraqi soldiers in their trenches and bunkers during two hours of heavy fighting. 

"It was fast, hard and vicious fighting," Simpson said. 

The shooting started only after it appeared the Marines would cross the bridge uncontested - a belief shattered by mortars fired as the forces approached the bridge. The Marines eventually succeeded in making their move toward the capital. 

The U.S. forces had also neutralized the Republican Guard's Medina division, while they were still fighting the Nebuchadnezzar troops. The Americans also attacked towns and positions north of Karbala, where 2,000 Fedayeen loyalists and Baath Party members were believed to be hunkered down. 

Three other divisions of the Republican Guard - Adnan, Hammurabi and Nida - were currently not engaged. 

At least 20 Iraqis were killed and an unknown number of fighters were taken prisoner, field reports said. No U.S. casualties were reported. One tank belonging to the Nebuchadnezzar Division was reported destroyed. 

"We have moved beyond where the Republican Guard is and beyond where the popularly known red line is," Thorp said. 

Army Lt. Col. Scott Rutter, a battalion commander, said Iraqi troops concentrated their attacks on his unit, allowing the rest of the brigade to pass through the Karbala gap unscathed. The gap is a chokepoint between a lake to the west and the city of Karbala to the east that opens onto a plain. 

Farther south, the highway leading out of the town of Nasiriyah was choked with coalition military convoys headed north. Vehicles churned up billowing clouds of dust as they drove through a barren landscape of cracked land dotted with green clumps of grass. Blown bunkers and collapsed buildings flanked the road. 

"The noose is starting to tighten around Baghdad," Sgt. Jeff Lanter, crew chief of a Marine CH-46E Sea Knight assault helicopter, said as he peered down at the mass movement. 

Around Karbala, Iraqi defenders fired anti-aircraft guns into the sky most of the night, as U.S. artillery pounded suspected military positions in the ancient town. B-52 bombers circled Karbala throughout the night, carpet-bombing some areas while fighter jets went after small targets. 

Pentagon officials have said the Republican Guard must be eliminated before ground troops move on Baghdad. For more than a week, coalition airstrikes and artillery barrages have pounded Republican Guard units to the south, west and north of the capital. 

Military officials have said that the Medina and Baghdad Divisions' fighting strength has been reduced by more than half. 

Meanwhile, an F-14 Tomcat fighter on a bombing mission in Iraq crashed late Tuesday because of mechanical failure and both crew members were rescued via helicopter, U.S. Central Command said. Neither was seriously injured. 

That brings to 67 the number of coalition forces extracted from hostile situations by search-and-rescue teams, the military said. 

The U.S. military would not give any further details of the accident. 

Also, a Marine Corps VA-8B Harrier jet was lost while coming in for a landing on the amphibious assault ship USS Nassau. The jet plunged into the water. The pilot ejected and was in fair condition, military said.