As many as 2,500 Republican Guard soldiers surrendered to U.S. forces, cable news networks reported, citing an unidentified military spokesmen at U.S. Central Command in Qatar. Sky News said the troops were from the Guard's Baghdad Division, which was defeated two days ago.

Iraqi resistance persisted after midday in some parts of Saddam International Airport, but U.S. officers were confident the takeover would be completed swiftly. More than 300 Iraqis were reported killed at the airport.

By the thousands, U.S. Marines and infantrymen converged on the outskirts of Saddam Hussein's capital. But American officials, both on the front lines and at the Pentagon, suggested an all-out assault might not be their first option.

Lt. Col. Scott Rutter, whose infantry battalion was trying to secure the airport, said the approach of U.S. forces should send a message to the people of Baghdad.

"We're here, and they can rise up and deal with the regime appropriately," he said.

Similarly, President Bush's top military adviser, Joint Chiefs of Staff chairman Gen. Richard Myers, said U.S. troops forces might isolate Baghdad, rather than storming it, while work begins on forming an interim, post-Saddam government.

"When you get to the point where Baghdad is basically isolated ... you have a country that Baghdad no longer controls," Myers said at the Pentagon. He estimated that Saddam's regime has lost effective control of 45 percent of Iraq's territory.

Captured Iraqi Republican Guard soldiers are marched by members of the 3rd Battalion, 5th Marine regiment after they took the town of Azaziyah. Associated Press photo

Iraqi Information Minister Mohammed Saeed Al-Sahhaf, in a statement attributed to Saddam, said U.S. forces would be crushed at the gates of Baghdad.

"We are determined, God willing, to defeat them and destroy them on the walls of our capital," the statement said. "Fight them, oh brothers, and hit them night and day."

At the airport, U.S. infantry sealed off the entrance closest to the capital, and combat patrols moved through the sprawling facility, which has a 13,000-foot runway and numerous military and civilian buildings.

Britain's defense secretary, Geoff Hoon, said the takeover of a large part of the airport was "a huge psychological blow to the regime."

"It demonstrates to the regime and the people of Baghdad that we're there," Hoon said.

In support of the ground troops, Navy warplanes from the carrier USS Kitty Hawk dropped satellite- and laser-guided bombs on hangars and a fuel depot at the airport, and struck a nearby military complex.

Coalition forces also bombed Iraqi Air Force headquarters in central Baghdad early today, one of many air assaults on the smoke-filled capital. Blasts on the outskirts before dawn shook buildings in the city center.

The air strikes came after the first widespread power outage of the war plunged Baghdad into darkness last night. Though some bombs hit the city before the blackout, U.S. military officials said they had not targeted Baghdad's power grid.

A sustained power outage would disrupt water supply and sewage, and create the threat of outbreaks of disease among the population of 5 million.

U.S. and British officials say they do not know whether Saddam is alive and well, wounded or dead.

Asked about Saddam by BBC radio, Iraqi Foreign Minister Naji Sabri replied, "He is well and the leadership is well and they are functioning as normal ... It is not our business to reply to rumors and lies."

The Marines' 1st Division was poised on the southeast outskirts of Baghdad after advancing along the Tigris River past abandoned Iraqi positions. Though temperatures headed into the 90s, Marines wore stifling chemical suits to protect against possible toxic attacks.

U.S. officials said control of the airport will allow American and British commanders to fly in troops, military equipment and supplies and could allow them to augment the relatively small number of ground forces outside the capital. It also could foil efforts by the Iraqi leadership to flee by air.

Collectively, the breathtaking speed of yesterday's gains, the sense — at least psychologically — of an allied encirclement of the capital, plus the absence of any well-organized Iraqi military defenses, left the impression that Saddam's regime may indeed be entering its final days.

Still, with doubts about the exact location of remaining Iraqi Republican Guard units, and with many paramilitary fighters said to be prepared to defend the capital, U.S. officials stressed that the war could be far from over.

"The regime has been weakened, to be sure, but it is still lethal," Defense Secretary Donald Rumsfeld said at the Pentagon. "And it may prove to be more lethal in the final moments before it ends."

There are no large Iraqi units coordinating a defense of the capital's outer ring, sources at U.S. Central Command said in Doha, Qatar. Two Republican Guard divisions, the Medina and the Baghdad, were effectively destroyed as cohesive operating units after Wednesday's fighting, according to U.S. officials, who said American troops have pounded the Iraqi forces with a relentless air and ground campaign.

"We can't tell who's in charge," Army Brig. Gen. Vincent Brooks said at a briefing at Central Command in Doha. "I don't think the Iraqi people can tell who's in charge either, and we have indications that the Iraqi forces don't know who's in charge."