U.S. Army troops led by tanks and armored vehicles overwhelmed Iraqi irregulars firing machine guns from pickup trucks today and, continuing a relentless U.S. advance, pushed on to seize an airfield near the sacred city of Najaf only 100 miles from Baghdad.

The resistance by Iraqi militiamen proved no match for the more than 70 M1 Abrams tanks and 60 M2 Bradley Fighting Vehicles of the 3rd Infantry Division's 2nd Brigade that clanked into the encounter and opened fire. But it tied up some of the unit's gunners for several hours. Afterward, mangled vehicles and the bodies of dead militiamen littered a dirt road within sight of Najaf, a city on the west bank of the Euphrates River that is the most prominent center of worship for Iraq's Shiite Muslims.

Along an earthen berm between a muddy plain and a road used by the advancing U.S. column, the bodies of four men wearing olive green uniforms lay where they fell, apparently waiting in ambush for U.S. forces. Nearby lay assorted military paraphernalia and ammunition, including rounds for a rocket-propelled grenade launcher.

One of the men wore shoulder boards indicating he was a senior officer, according to a U.S. soldier who happened upon the bodies after participating in a recovery operation to pull out tanks and armored personnel carriers that got stuck in an adjacent marshy plain.

U.S. officers said about 15 Iraqi vehicles were destroyed and about 100 fighters killed, with 20 captured. They said tanks and Bradleys attacked the militiamen's vehicles, while dismounted infantry troops went after the ambushers. The brigade also has snipers, equipped with high-powered scopes and night-vision devices, who have been seen training their rifles on targets in and around suspicious vehicles since the brigade arrived in this area Saturday.

As the battle against the militiamen wound down, a force of about 30 Iraqi BMP armored fighting vehicles attempted to attack the U.S. column from the north. Bradleys of the division's 3rd Battalion, 15th Infantry Regiment were gearing up to counterattack, but a combination of airstrikes by A-10 Thunderbolt tank-killing planes and AH-64 Apache helicopters, plus artillery fire from Paladin 155mm howitzers, finished the job before the U.S. armored personnel carriers, mounted with a 25mm gun that fires depleted uranium rounds, could move into position.

U.S. commanders said the Iraqi leader, President Saddam Hussein, was apparently trying to use the ill-equipped and poorly trained militiamen as a guerrilla force against the American forces, whose mission is to oust him from power. Although the tactic has slowed U.S. and British forces in southern Iraq, U.S. commanders here, about 200 miles north of the Kuwaiti border, described the fighting as a mismatch that would not deter them.

"Today we attacked through this area to clear remnants" of the militia group, said Lt. Bevan Stansbury, 27, of Austin, Tex., the executive officer of the 3rd Battalion's Bravo Company. He said units were still on alert because the militiamen "have been probing the brigade's lines tonight." He described the resistance so far as "weak."

In racing up north from the Kuwaiti border to threaten Baghdad, parts of the 2nd Brigade have covered about 700 miles in three days of hard driving.

"We're going to move in a couple days what in World War II it took six months to do," said Col. David Perkins, the 2nd Brigade commander, before launching the invasion from Kuwait. He said one element of the force would move farther in one day than U.S. forces in the Persian Gulf War moved in three days.

Perkins likened the challenge of the road march to that faced by Hannibal when he surprised the Romans by crossing the Alps with elephants in 218 B.C. In this case, however, "the Alps are wadis, and the elephants are 72-ton tanks," he said, referring to the dry riverbeds that are common in Iraq.

Bravo Company seized the airfield, southwest of Najaf, without opposition, Stansbury said, and was occupying it with Bradleys. As night fell, warplanes could be heard crisscrossing the skies over the Najaf area, apparently conducting a bombardment of Iraqi infantry positions reported to be preparing a defense in the Karbala area about 50 miles north of here.

For hours today, 3rd Division armor and assorted other vehicles moving north passed within sight of Najaf, site of the tomb of Ali, the son-in-law of the Prophet Muhammad who Shiites believe was his rightful successor. After the Gulf War in 1991, Shiites in the southern part of the country revolted against Hussein's government, but the U.S. assistance they expected never came, and the rebellion was brutally suppressed.

An Iraqi man waves as he greets U.S. soldiers attached to the 2nd Battalion, 3rd Brigade Combat Team, of the Army's 3rd Infantry Division, now positioned in southeastern Iraq.Army 3rd Infantry Division soldiers secure a field near Najaf, Iraq, at sunrise.