American forces have set up checkpoints on all the major roads into and out of Baghdad and are conducting armed reconnaissance missions into the city proper, reports CBS News Correspondent David Martin.
The first foray into Baghdad was carried out by tanks and armored vehicles of the Army's 3rd Infantry, and there are estimates as many as 2,000 Iraqis were killed. But the Americans suffered some casualties, too – at least one dead and several wounded. They left one damaged tank behind and much of the city still in control of forces loyal to Saddam Hussein, who released another tape supposedly showing him and his two sons still in command.
In southern Iraq, British forces moved into Basra, their first major incursion since encircling the city early in the war. Three British soldiers were killed. An unknown number of Iraqi paramilitary fighters were killed and others taken prisoner.
In other major developments:
A U.S. C-130 transport plane landed at Baghdad's international airport Sunday, carrying unknown cargo but weighted with symbolism and tactical importance. The arrival presaged a major air resupply effort for U.S. troops, dependent until now on a tenuous line stretching 350 miles to Kuwait.
U.S. officials declared Baghdad cut off from the rest of Iraq.
"We do control the highways in and out of the city and do have the capability to interdict, to stop, to attack an Iraqi military forces that might try to either escape or to engage our forces," said Gen. Peter Pace, vice chairman of the Joint Chiefs of Staff.
Assorted prizes fell into allied hands, some after hard fighting, but U.S. forces had yet to confront Baghdad's last-ditch defenders on a large scale.
"They are extremely weakened, but that does not mean they're finished," Pace said of the Republican Guard.
Pace said the Republican Guard's main weapons systems are gone and the force probably cannot assemble more than 1,000 men in any one place.
Capitalizing on their dominance of the skies, U.S. commanders began deploying planes over Baghdad 24 hours a day, ready to direct strike aircraft to ground targets.
Southeast of Baghdad, Marines seized one of Saddam's palaces, poked through remnants of a Republican Guard headquarters and searched a suspected terrorist training camp, finding the shell of a passenger jet believed to be used for hijacking practice.
A statement aired on Iraqi TV in Saddam's name was typically defiant but hinted at problems coordinating the nation's defense. It urged soldiers who had been separated from regular units to join up with any unit they could find.
On another vital front, British troops thrust to the center of Basra, Iraq's second largest city, with a sense they were finally shaking Saddam loyalists loose.
British Desert Rats went into the city of 1.3 million with more than three-dozen tanks and armored cars. But they found resistance softer than expected, picked up reports that the local Baath Party leadership was crumbling and fought into the core, losing at least three soldiers and finding their arrival cheered by hundreds of citizens.
"We have a lot of it occupied," British Maj. Gen. Peter Wall told reporters. He said it might take days to put down renegades.
In chalking up military gains, the U.S. accelerated a campaign of persuasion, too, aimed at getting the Republican Guard to surrender. And Washington's attention began turning to postwar Iraq.
Pace said the U.S. would welcome Republican Guard commanders and troops in a postwar government if they surrendered now.
"I mean, there's a small clique around Saddam Hussein who are the perpetrators of all the crimes against humanity," Pace said in a broadcast interview.
"Below them are still many senior leaders and troops who have their free will to decide what their life is going to be like. They can surrender and become part of the future free Iraq, or they can fight and die."
The U.S. is deploying some Iraqi exiles and dissidents around the country to help root out pro-Saddam elements, keep order and distribute aid, according to one such organization, the Iraq National Congress. The group said several hundred of its members were flown to an area near the city of Nasiriyah.
In and around Baghdad, the intensified ground fighting was taking a toll on civilians. CBS News Correspondent Lara Logan reports many are fleeing the city, people and possessions crammed together as they make their escape.
Staff at the al Kindi hospital have been overwhelmed by a flood of casualties. One young girl was orphaned when a missile hit her family home in one of the city's poorest neighborhoods.
"When they attack us I burn, I burn" she cried.
Her younger sister was also burned along with her brother, whose injuries were too severe to show. The hospital is struggling without electricity and is ill equipped to deal with burns. Among the patients are Arab volunteers injured in fighting against U.S. forces.