The U.S. military's bombardment of Najaf escalated sharply today, but the assault is proving problematic for the Army, which finds itself entangled in precisely the sort of urban combat that military planners hoped to avoid.

Continuous ground fire and airstrikes battered suspected military targets barely a half-mile from one of the holiest sites for Shiite Muslims, the tomb of Ali, son-in-law of the prophet Muhammad. Within the city, according to Army intelligence estimates, there are 1,400 to 2,100 Iraqi fighters, made up of Saddam's Fedayeen and Al Quds militias.

Army Special Forces teams operating around Najaf said today that Fedayeen militiamen are converting the Tomb of Ali into a central stronghold, firing rocket-propelled grenades, mortars and automatic weapons from the narrow alleys and neighborhoods around the shrine, which is also adjacent to a market. "It's a rabbit warren," one commander said.

Rooting Iraqi defenders from the shrine is a difficult tactical problem as well as an enormous political challenge, and senior officers worry that the operation foreshadows fights in the many urban areas leading to Baghdad. Commanders so far have tried to minimize collateral damage, although the number of civilian casualties in Najaf -- a city of more than half a million -- is unknown.

The 101st Airborne Division has attacked Najaf from north and south to secure U.S. supply lines leading toward Baghdad. Today, five GBU-12 bombs dropped shortly after 3 p.m. (7 a.m. EST) by U.S. Navy F/A-18s ripped through a tree line below the steep slope on which the besieged city sits, obscuring the tomb's gilded mosque dome with billowing black smoke.

Viewed from a U.S. Army command post a mile west of town, today's attack was a relentless choreography of fire against defensive trenches and bunkers lining a canal on the western edge of Najaf. The boom of M1-A2 Abrams tank guns punctuated the rattle of .50-caliber machine guns and Mark 19 grenade launchers. Foaming brown smoke from detonating mortar shells boiled through a palm grove, followed by the white blossoms of 105mm artillery rounds.

TOW antitank missiles exploded with orange flashes below the roofline of what appeared to be an apartment building, while OH-58 Kiowa helicopters flitted above the city at 70 mph, ripping buildings and defensive pits with rocket and machine-gun fire.

Col. Ben Hodges, commander of the 1st Brigade of the 101st, gestured with his field glasses toward the smoking trees below the escarpment in the middle distance. "We are under no time pressure. . . . There are villages in that wood line, so we can't be indiscriminate. But I'm probably pushing it more than I would have two weeks ago."

In a conference over the hood of a Humvee during the airstrikes, a Special Forces commander today told a senior Army officer, "Sir, we don't want a war of attrition, but we are in one."

"We are," the officer agreed. "It's a siege."

More heartening to Army commanders is that air attacks have continued to take their toll on the Republican Guard. The Medina Division has long received special attention from planners because it blocks the southern approaches to Baghdad and because it is considered the best equipped of the Republican Guard units. An intelligence officer said airstrikes may have cut the Medina's combat effectiveness in half, and one particularly battered brigade is believed to be down to 20 percent.

Some Republican Guard units are repositioning, Army sources said, and the Iraqis have been particularly aggressive in pushing artillery south to support paramilitary fighters in the Euphrates Valley. The 1st Adnan Mechanized Division of the Republican Guard, which is usually based in northern Iraq, is believed to be moving south toward Baghdad, suggesting that the recent parachute drop into northern Iraq by the lightly equipped 173rd Airborne Brigade has had a limited effect in freezing Iraqi forces.

More difficult for the Army in the short term is how to secure Najaf and other cities on the road to Baghdad without worrying about attacks from the rear. The 250-mile supply line angling from Kuwait to central Iraq is already vulnerable to both guerrilla raids and more concerted attacks. Occupying an urban area the size of Najaf will likely require considerable forces, both to guarantee security and to help distribute humanitarian aid.

But the Army has only two divisions in Iraq, the 101st Airborne and 3rd Infantry, supplemented by a brigade from the 82nd Airborne Division, which currently is committed to supply line security. The lightly armed 2nd Cavalry Regiment is expected to arrive soon, and the 4th Infantry Division also has begun pushing into Kuwait. The 101st earlier today ordered an airstrike to destroy a canal bridge east of the Euphrates to forestall an attack in the flank by Iraqi reinforcements from Baghdad.

The escarpment battle was one of three simultaneous fights conducted today by the 101st. Twenty miles north of Najaf, the 2nd Brigade found itself in a stiff fight against Iraqi artillery, and an infantry battalion that might have included troops from the Medina Division, Army sources said.

One U.S. soldier was shot and killed while riding atop a tank. He was hit in chest by a round that penetrated a crease in the side of his body armor.

In the same firefight, which occurred along the southern approaches to the Euphrates River town of Hilla, eight AH-64 Apache attack helicopters from the 101st received damage from small-arms fire or 57mm antiaircraft guns, and one pilot was slightly wounded, according to Col. Gregory P. Gass, commander of the 101st Aviation Brigade. Damage to two of the Apaches was substantial, but all returned safely to their base, and all can be repaired, Gass added.

The third fight was on the southeastern edge of Najaf. As one battalion from the 1st Brigade pushed farther into the city under light mortar fire and cleared a military training compound, another battalion swept east to capture the city airfield. Riflemen with fixed bayonets fanned out across the airport perimeter, engineers searched for mines and two enormous D-9 bulldozers cleared the 11/2-mile long runway of barrels, culvert pipes and other debris scattered by Iraqi defenders.

Loudspeakers blared messages in Arabic to the small farms and hamlets between the airfield and the Euphrates: "Stay away. Remain in your homes." Trolling Kiowas, at treetop level, shot up military trucks camouflaged with palm fronds. When one truck was riddled with .50-caliber rounds, "at least a dozen missiles 12 to 15 feet long ignited and came off the front end of the truck," said Lt. Col. Stephen M. Schiller, the Kiowa battalion commander.

Schiller and other Kiowa pilots also discovered and destroyed a large ammunition cache east of the airfield. The percussion of secondary explosions rolled across southern Najaf for more than a half-hour. Another Kiowa attack on a building in central Najaf left dense black smoke over the city for much of the afternoon. Three Kiowas suffered light damage from ground fire on Sunday; it was unclear whether any were hit today.

The fighting took on an increased tempo today in the 101st Airborne's sector; there were mine strikes on a tank and Humvee, squatting groups of flex-cuffed prisoners in concertina-wire cages, and a flurry of hurried conferences over the hoods of various Humvees. Through it all, Iraqi civilians could be seen trying to get on with their lives, whether working in the onion fields around the airfield or trudging down dusty lanes with water jugs on their shoulders.

Said one senior officer, "It's really a war now."

A hooded Iraqi POW comforts his 4-year-old son at the 101st Airborne Division's regrouping center near Najaf. The military did not want to separate the two.At a refueling point for the 101st Airborne Division in southwest Iraq, two crew chiefs look over the cracked canopy of an Apache attack helicopter that was hit by small-arms fire while flying over Najaf.