Artillery and rocket barrages set buildings on fire and raised a pall of thick, black smoke over Nasiriyah on Friday as Marines outside the Euphrates River city tried to stamp out Iraqi resistance on a key supply route to Baghdad.

At least some units at the front end of the lightning advance toward the capital have been ordered to halt for what officers in the field on Saturday called an operational pause to give them a chance to resupply. The closest U.S. forces are about 50 miles south of Baghdad.

A battalion of Marines set up makeshift camps on the side of a road in central Iraq, waiting for badly needed fuel supplies and working to improve communications with units further back.

"We're trying not to screw ourselves up and to watch our rear by establishing these lines of communication," a lieutenant in the battalion said.

In central Iraq on Friday, a soldier was killed and five injured when their Bradley fighting vehicle rolled over, the Central Command said.

Last week's sprints across the desert have turned into a distant memory, with almost perpetual traffic jams of supplies and troops on roads north and continuing attacks by Iraqi irregular forces. Overnight, Army troops outside Karbala, 50 miles southwest of Baghdad, fended off Iraqi harassment of their positions. Aircraft and artillery battered the city and targeted Iraqi troops moving south to confront the Americans.

At the same time, U.S. and British warplanes and missile strikes were trying to wear down the fighters protecting the approaches to Baghdad: Saddam Hussein's best fighters, the Republican Guards.

In the 101st Airborne Division's first known offensive mission of the war, Apache helicopter gunships hit Guard tanks and installations south of the capital. Two Apaches crashed in swirling sand as they returned to the 101st's base in a remote part of the Iraqi desert, but all crewmembers escaped injury.

In Iraq's second largest city, Basra, U.S. warplanes firing laser-guided missiles destroyed a building where 200 paramilitary fighters were believed to be meeting. The paramilitaries, known as Saddam's Fedayeen, have kept a fierce grip on Basra even as British forces encircle the city.

The battle over Nasiriya, meanwhile, gave a sample of the kind of firefight that may await coalition forces in Baghdad, 200 miles to the north.

All day, Marines fought pockets of Iraqi fighters. Cobra helicopters fired rockets into the city, raising plumes of white smoke. Artillery and tank fire rumbled, sparking bursts of white flame low on the horizon. Helicopter crews drew almost continuous small arms fire and rocket-propelled grenades.

Four Marines with the 1st Expeditionary Force were reported missing.

Heavy smoke from a burning power plant poured over the city of 500,000, and other buildings were also on fire. In Nasiriyah's eastern neighborhoods, some buildings were reduced to shells, with debris scattered in the streets.

Nasiriyah -- at a junction of highways and leading up to Baghdad -- was just one of the places were U.S. forces were trying to ease the way as commanders try to mass troops and supplies for on the capital's doorstep.

"We'll attack when we're ready," Air Force Gen. Richard Myers, chairman of the Joint Chiefs of Staff, said when asked when the assault on Baghdad would begin.

At least 1,200 troops are in place in northern Iraq, and special operations forces have secured air fields and other strategic targets in the west.

Marine units pushed forward. Instead of trying to avoid engagements, they looked for them, trying to clean out pockets of regime loyalists as they go. Convoys moved day and night, taking food, fuel and other supplies north, traveling in total darkness without headlights.

It's "blue-collar warfare," said Lt. Col. B.P. McCoy, commanding officer of the Marines 3rd Battery, 4th Regiment. "There's no magic solution to it. It is just the hard-grinding work of patrols."

But they did advance. Authorities said the 1st Marine Expeditionary force was north of Qalat Sikar, 50 miles up the road from Nasiriyah.

Behind them, some pockets of resistance were vanquished. Authorities said the port of Umm Qasr was secured after days of sporadic fighting, allowing the delivery of humanitarian aid for desperate Iraqis.

Elsewhere, battles raged. At Diwaniyah, a Marine died and another was wounded in fighting with irregular Iraqi forces at a cement plant. In Basra, Friday night's airstrike appeared aimed at helping break the grip of Saddam's Fedayeen on the city. Central Command said it didn't know what happened to the occupants of the targeted building. Earlier in the day, the paramilitaries opened fire on civilians trying to flee the city.

Saddam's Fedayeen were spotted in and around Nasirayah. Some of the Iraqis wore uniforms and others were in civilian clothes, riding in white pickup trucks and taxis. They waved white T-shirts, and then started shooting.

On Thursday, a CH-46 Marine transport helicopter trying to pick up casualties and deliver supplies to Marines in battle was forced to turn back when it came under fire from small arms and rocket-propelled grenades.