U.S. military forces intensified pressure on Shiite Muslim militiamen early Tuesday around the shrine of Imam Ali, with cavalry units mounting armored attacks to the north and east of the holy site while Marines exchanged fire with guerrillas to the west.

In an unusually active night of offensive operations, explosions echoed across Najaf and black smoke from heavily shelled buildings billowed over the shrine's brightly lit minarets and signature gold dome.

At one point shortly after midnight, the view from the western edge of the vast cemetery north of the shrine was a tableau of organized violence. In the sky to the left, a 155mm illumination round hung in the air, fired by Marine howitzers to light an area at the behest of Iraqi police. Straight ahead, cavalry armor fired volley after volley of red tracer fire at several tall buildings just inside the road that rings the shrine. Each burst of 20mm shells exploded in a golden flurry on impact, igniting a fire that soon engulfed much of the area in smoke.

The sound, like steady knocking on a door, was later overtaken by the chain gun of an AC-130 Spectre gunship, which destroyed a booby-trapped roadblock. AH-64 Apache attack helicopters hovering behind a nearby ridge fired volleys of Hellfire missiles.

To the east of the shrine, armor from another cavalry outfit was involved in what a commander described as heavy fighting for control of a complex of buildings.

Meanwhile, in Baghdad's heavily Shiite Sadr City neighborhood, an explosion killed four people and injured nine, Qasim Saddam, the director of Sadr Hospital, told the Associated Press. The cause of the blast was unclear, and the U.S. military said it was unaware of the incident.

In Fallujah, in the restive Sunni Muslim region west of Baghdad, U.S. warplanes reportedly carried out an airstrike early Tuesday. Witnesses told the AP that it was unclear what the target was, but they reported flames and smoke in southern neighborhoods.

The raid on central Najaf from the north was the first in which U.S. forces penetrated the jumbled neighborhood immediately surrounding the shrine. It brought little resistance from the Shiite militia force, the Mahdi Army, and commanders of units waiting in the nearby cemetery said they were surprised to have encountered no fire at all.

"I think they're tired of us beating on them, but it doesn't mean they've given up," said Army Lt. Col. Myles Miyamasu, commander of the 1st Battalion of the 1st Cavalry Division's 5th Regiment, which mounted the raid.

The morale of the Mahdi Army is of great interest to U.S. commanders and the Iraqi officials who approve or reject major combat operations. Officers say they have intelligence indicating significant numbers of the Shiite volunteers, who are loyal to rebellious cleric Moqtada Sadr, have either been killed or retreated.

"We hear from multiple sources there are some guys drifting away, but at the same time there seems to be no shortage of dedicated guys up there," said one U.S. commander.

"We have many killed and wounded and we cannot count them because of this situation," said Ahmed Shaibani, a Sadr aide, who appeared tired. He said Sadr's organization was burying the fighters because it could not get the corpses to the families, many of whom are in Baghdad, a major recruiting center for Sadr.

No movement was reported in the sporadic negotiations among Sadr's representative, mediators and the Iraqi government.

With no end to the fighting in sight, civilians in the area around the shrine are feeling the effects of the conflict.

"I want the whole world to see my situation," said a mother of two, who wore dirty clothes because water and electrical power have been shut off in her neighborhood. "What have we done that they do such a thing to us? We don't have food and water, and my husband is sick. Oh God, what did I do?"