KUWAIT, March 24 - Having swept more than 300 miles over desert and mud in southern Iraq, an American army gathered itself south of Baghdad today, flinging artillery fire and airborne helicopter forces against the Medina Division of Iraq's Republican Guard, the outer ring of Saddam Hussein's final defense.

But withering antiaircraft fire described as a ``hornet's nest'' drove back an airborne force of more than 30 Apache helicopters, downing one in a farmer's field.

Iraqi television showed images of what it said were the two pilots. The Pentagon identified the missing crewmen as Chief Warrant Officer Ronald D. Young Jr., 26, of Georgia, and Chief Warrant Officer David S. Williams, 30, of Florida.

Nearly all the gunships sustained damage, and military commanders said they were disappointed in their initial failure to destroy the entrenched Iraqi forces from the air.

In Nasiriya, in central Iraq, 5,000 marines battled to cement control over the city, the scene of intense fighting on Sunday that left several marines and soldiers dead, and others wounded, captured or missing. Control of the Euphrates River crossroads and its bridges is vital to supporting and provisioning the army that raced over the river toward Baghdad this weekend.

``This will plainly be a crucial moment,'' Prime Minister Tony Blair of Britain said of the battle that began Sunday night and carried into today.

Elsewhere, both Secretary General Kofi Annan of the United Nations and President Vladimir V. Putin of Russia expressed concern over a looming civilian crisis in Basra. There were intense efforts by the American-led invasion force to secure the port at Umm Qasr and clear mines so that an aid ship could bring relief supplies ashore.

In northern Iraq, an American bomber trying to take out a bridge reportedly hit a bus at the Syrian border, killing five civilians and wounding many more.

At a briefing in Qatar, the commander of the coalition forces, Gen. Tommy R. Franks of the United States Army, asserted that Sunday's casualties and setbacks were no surprise. Some analysts have expressed concern that the Army is strung out with inadequate protection for supply lines.

General Franks, speaking at the Central Command base in Qatar, said that coalition forces had intentionally bypassed enemy formations in a rapid dramatic drive toward Baghdad, though he said they had still managed to take 3,000 prisoners.

He spoke a day after American and British ground forces suffered their worst casualties so far - at least 20 American soldiers were missing or killed and 50 or more were wounded - battling determined forces on two fronts in what an American commander said were ``the sharpest engagements of the war.''

General Franks said fedayeen fighters - members of militia groups under the control of Saddam Hussein's son Uday - had been harassing American rear positions in southern Iraq.

``We know that the fedayeen has in fact put itself in a position to mill about, to create difficulties in rear areas,'' the general said, ``and I can assure you that contact with those forces is not unexpected.''

During his briefing, General Franks repeatedly emphasized that nothing he had seen so far had surprised him or was unexpected.

Asked if he and his commanders had underestimated the tenacity of Iraqi forces and their ability to communicate despite attacks on their command-and-control sites, he replied, ``There is nothing unexpected about it.''

General Franks also said he thought that Basra and Umm Qasr would be pacified ``within a very few days'' to allow the entry of humanitarian supplies. Late last week, American and British commanders said that both cities were under allied control.

The general also said that he was not surprised by the prevalence of lethal friendly fire nor that American forces had so far found no chemical or biological weapons.

American officials reported that one soldier in the Third Infantry Division was killed by a sniper today as the division consolidated its positions on a broad plateau north of Najaf.

After pushing deep into Iraq over three days, the division's units continued to clash with small pockets of Iraqi forces in and around their positions. There were few indications that the division's heavy armored brigades were prepared to move forward.

Instead, the American soldiers concentrated on establishing security over dozens of miles of scrub desert and on bringing up food, water and other supplies after a long march northward from the Kuwaiti border.

Army weapons inspectors began searching an ammunition depot seized on Sunday for evidence that Saddam Hussein's government had stored chemical weapons there. The First Brigade's second battalion seized the sprawling depot without a fight, capturing 93 prisoners, including one believed to be a general.

The depot - two-and-a-half miles by five-and-a-half miles in size - is on a Bush administration list of suspected chemical weapons sites, and it was one of the main objectives of the division's rapid advance over the escarpment north of Najaf.

Most of the American losses have occurred at Nasiriya, which is about 100 miles north of the Kuwaiti border. An Army maintenance convoy was ambushed there on Sunday, resulting in the death or capture of about a dozen soldiers, some of whose images were beamed around the world by Arab television. Fighting continued there today.

Later on Sunday, marines attacked the city, and during a fierce battle with about 500 Iraqi defenders, a rocket-propelled grenade struck a troop carrier, witnesses said, killing up to 10 troops.

Military officials said dozens more were wounded in the battle, and helicopters were seen ferrying the wounded out of the city.

Meanwhile, the main force of the allied army - the Army's Third Infantry Division and the First Marine Expeditionary Force - raced toward Baghdad, with the largest Army force passing Najaf. It paused on the way to Karbala for the night, about 100 miles south of the Iraqi capital, Baghdad.

In general, allied forces skirted centers of population, keeping their focus firmly on Baghdad. The central American objective of the war is the removal of Saddam Hussein's government from power and the subsequent disarmament of the country.

President Bush said he grieved with the parents of the soldiers lost in battle but he expressed confidence that the campaign would succeed.

General's Franks's deputy, Lt. Gen. John Abizaid, raised a number of alarms about the days ahead. Intelligence reports, he said, indicated that Iraqi troops were wiring explosives to bridges in Baghdad and placing other explosives ``against certain buildings in Shia neighborhoods,'' implying that the Iraqi government might be preparing to inflict civilian casualties and then blame allied bombing for the damage.

He also described scenes from today's fighting that appeared to enrage American commanders.

In one incident, Iraqi troops raised a white flag of surrender - only to attack their approaching captors with artillery fire. In another, he said, a group of civilians made motions to surrender and then opened fire when American marines came forward. Five marines were reported wounded.

Over all, the Iraqi tactics, while showing little coherent military organization, appeared to expose a potential weakness in the dash the American forces were making to Baghdad.

By skipping over cities, American forces appeared to have left their flanks and rear areas exposed to counterattacks by irregulars under the command of Republican Guard officers dispatched by Baghdad to galvanize resistance and slow the coalition advance.

Among the missing Americans were a dozen members of the Army's 507th Maintenance Company who took a wrong turn and drove into Nasiriya without an armored escort, military officials said. Their convoy was attacked by tank fire.

Iraqi television reports said a number had been taken prisoner, and Pentagon officials said that appeared to be true. State television broadcast interviews with five of the captives, which was rebroadcast around the world by Al Jazeera, the Arab satellite network based in Qatar.

The American prisoners appeared frightened. Some bore wounds. An interviewer asked them to state their names and their units. Other video showed the bodies of several soldiers as a smiling Iraqi rolled one body over to face the camera.