U.S. forces were trying to wrest control of the Rashid military airport in southeast Baghdad Tuesday but they were faced with some unconventional, and sometimes intense, resistance. Parts of the airport, however, were seized with little difficulty.

Meanwhile, U.S. forces are escalating their attacks on Saddam Hussein and the close friends and relatives that form his inner circle.

U.S. Marines are on the grounds of the Rashid Airport, located about 3 miles from the center of Baghdad, after destroying Iraqi tanks and armored personnel carriers to cross the Diala River, Brig. Gen. Vincent Brooks said Tuesday.

"There was resistance, most of it leading to it, and there's also resistance beyond it," Brooks said during a U.S. Central Command briefing in Doha, Qatar.

Reporters entering offices found military maps covered in circles and lines, along with what appeared to be photographs of senior Iraqi military officers hanging on the walls.

"We are just securing it, making sure there are no enemy forces left in it that might be straggling behind," U.S. Captain Matt Watt of the 1st Marine Division's Lima Company told Reuters.

The airport is in a "militarily significant" area between the Diala and Tigris rivers, he said. Taking the airfield would aid efforts to secure the Iraqi capital -- and prevent the escape of high-ranking Iraqi officials -- as forces loyal to Iraqi President Saddam Hussein turn increasingly to fierce urban combat, Brooks said.

U.S. soldiers told Reuters reporters there that Iraqi morale seemed to have collapsed during their advance on Baghdad.

"Maybe at first they had high spirits and thought they could do a good job," said Staff Sgt. Mark Prewitt, speaking from the turret of his tracked armored vehicle, one of a group guarding the airfield perimeter.

"Since we pushed forward at a high rate of speed, they are realizing that it's not worth it, they are probably realizing they are fighting for a lost cause."

During the fighting, a coalition A-10 "Warthog" warplane was shot down by a surface-to-air missile. The pilot ejected safely and was recovered by U.S. ground forces near the airport.

Although Central Command said it was investigating the incident, an official there said it appears that the plane was shot down by an Iraqi missile, and not friendly fire.

U.S. forces are also fighting north of Baghdad, with units of the Army's V Corps, with small pockets of resistance elsewhere in the capital and east of Karbala.

Some of the fighting, Brooks warned, is intense urban combat that can result in deaths and injuries to civilians and bystanders.

"We know that as we conduct operations inside of Baghdad we should anticipate attacks from unexpected locations, that some of the military actions might be unconventional in nature, whether it's the use of car bombs or whether it's ambushes, the use of snipers, or certainly the consistent pattern we've seen elsewhere of using civilians as shields," Brooks said.

"We can only be reminded that the risk increases for the population as we do these operations," he added. "But we have to remain focused on our objective of removing this regime before there's greater loss of life."

At times, the Iraqi forces include formations of 20 to 60 vehicles, including T-72 tanks or civilian trucks outfitted with military weapons. "Often, all of those vehicles are destroyed -- any vehicles that are encountered," Brooks said.

Parts of the V Corps remained in the center of Baghdad overnight. Other V Corp units carried out attacks Tuesday morning from the south and from the north of the city -- battling Iraqi tanks, armored vehicles, artillery and armed civilian vehicles.

It remained unknown whether Saddam and his sons were alive. On Monday, a U.S. warplane dropped four bunker-buster bombs and blasted a smoking crater 60 feet deep at a restaurant where they were believed be meeting.

Even before Monday's bombing, Iraq's leaders were finding it difficult, if not impossible, to direct troops and other government loyalists, Pentagon officials said.

"We may not know if or where he is," Defense Secretary Donald Rumsfeld said of Saddam before news of the Mansour strike broke, "but we do know that he no longer runs much of Iraq."

Pentagon officials said Tuesday that it could be days before they know if Monday's bombing aimed at Saddam was a success. But they hope to cut off Saddam and his advisers from the rest of the country, if not kill or capture them outright.

The Associated Press contributed to this report.