One group of Marines traded fire with Iraqi forces Friday amid the burning buildings of An Nasiriyah, while others pushed north of the strategic city in a grinding bumper-to-bumper movement of weapons and supplies.

Clashing repeatedly with Iraqi fighters in front and behind, the convoy was moving and night, headlights out, on a fighting drive north toward Baghdad.

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

At least one American was reported killed in the fighting. Two other Marines were killed after they were accidentally run over by one of their vehicles as they slept.

At both the southern city of An Nasiriyah and Ad Diwaniyah in the south-central part of the country, the Marines called in Cobra helicopters and other aircraft to pound Iraqi ground forces -- attacks made possible after a two-day sandstorm finally let up.

In An Nasiriyah, Marines and Iraqi forces exchanged tank and artillery fire in a clash that set the power plant and other buildings on fire and cast thick black smoke over the town.

A CH-46 Marine transport helicopter was forced to turn back after being fired on while trying to pick up casualties and deliver supplies to Marines fighting in An Nasiriyah. Iraqis fired small arms and rocket-propelled grenades.

An Nasiriyah, located on the Euphrates River near a junction of roads that lead from Kuwait to Baghdad, has been the scene of some of the fiercest fighting of the war, prompting Marines to call the southern entrances to the city "Ambush Alley."

Earlier this week, more than 25 Marines were wounded in fighting at An Nasariyah, and U.S. officials said some or all of them were hurt when one Marine unit mistakenly fired on another.

The continued battle for control of the city comes just days after Army units pushed on ahead toward Baghdad, leaving it to the Marines to secure the rear.

Marines flying over the area have encountered bursts of anti-aircraft fire. The Marines have also faced pockets of resistance from Iraqis in uniform and from people who looked like civilians in white pickup trucks and taxis. These Iraqis waved white T-shirts, then started shooting. The Fedayeen, Saddam Hussein's hard-core loyalists, have also been spotted.

To the north of An Nasiriyah, Marines protected by tanks, artillery and air cover pushed north toward Baghdad with food, fuel and other supplies, trying to clean out irregular Iraqi forces along the way.

The Marines waged a firefight at a cement plant seized by irregular Iraqi forces near Ad Diwaniyah, field commanders in the area said. One Marine died and one was wounded in the fighting.