Behind a withering aerial assault on Iraqi defenses, U.S. Marines and Army troops launched a two-pronged assault on the Republican Guard divisions defending the approaches to Baghdad, ending a week-long pause in the U.S. push toward the seat of President Saddam Hussein's government.

Columns of M1 Abrams tanks and armored vehicles from 1st Marine Division moved out of staging areas, where the Marines had rested and resupplied, and headed into the outer defenses of the Republican Guard's Baghdad Division around the city of Kut, about 100 miles southeast of the capital. This morning they seized a key bridge across the Tigris River, a Marine officer said, and took control of the main highway from Kut to Baghdad

To the west, north of Karbala and about 50 miles south of Baghdad, units from 7th Cavalry Regiment of the Army's 3rd Infantry Division were engaged in a "knock-down, drag-out" battle with elements of the Republican Guard's Medina Division, a Pentagon official said. The nearby 101st Airborne Division seized a portion of the city of Najaf before moving to support the assault on the Republican Guard.

"We're tightening the noose around Baghdad," Lt. Col. George Smith, a top war planner for the Marines, said as the troops began moving out Tuesday.

Taken together, U.S. commanders said, the Marine and Army movements represent the beginning of the battle with the main defensive deployments around the capital that U.S. officers have depicted as a decisive chapter of the war to dismantle Hussein's three-decade-old Baath Party government. The Republican Guard are considered the bulwark of Hussein's defense, far better trained and equipped than the regular army units encountered in the south.

The ground move against the Republican Guard was accelerated by a day after Army units engaged in skirmishes with Iraqi units on the southern edge of the Baghdad defenses on Monday. That clash began as a "probing action" of Iraqi defenses, the Pentagon official said, but grew into "an all-out battle." The Marines' move at Kut and the full Army thrust south of Baghdad were then ordered to begin right away.

The troop movements were preceded by a day of relentless air attacks and artillery and rocket barrages against Iraqi troops arrayed in defense of Baghdad. All afternoon Tuesday, the contrails of U.S. warplanes were seen hanging over the area around Karbala, white streaks across the clear sky over central Iraq.

F/A-18 Hornets and other warplanes from the 3rd Marine Air Wing struck Iraqi T-72 tanks in reinforced shelters, as well as armored personnel carriers, fuel trucks, artillery batteries, bridges, missile canisters, radar systems, a cable repeater station and a building believed to be a Baath Party office, Marine officers said. "We're hitting everything we can," said Maj. Glindon Ashbrook.

Six large explosions were heard in Baghdad early this morning, in a resumption of a bombing campaign against the capital that had subsided slightly. Several cruise missiles were fired Tuesday, however, at sites in Baghdad and to the southwest. Among the targets in Baghdad, U.S. officials said, was a complex that serves as the office of the Iraqi National Olympic Committee, which has been run by Hussein's elder son, Uday, and according to Iraqi exiles houses a prison run by him.

The heavy bombing hit a residential district in the town of Hilla, along the Euphrates River in central Iraq, killing at least 11 civilians, most of them children, the Reuters news agency reported. A reporter taken to the town by government officials counted 11 bodies of civilians and two of Iraqi fighters.

Going on the Ground

The renewed U.S. troop movements were depicted by U.S. military officials as the first stage of an assault on the well-entrenched defenses ringing Baghdad and the largest ground offensive since the invasion began 13 days ago. U.S. military officials emphasized that the attack was made possible by the massive aerial bombardment of Republican Guard artillery, tanks and barracks over the past few days. Although Iraqi commanders have shifted in reinforcements from the north, they said, the pounding from the air has left two of the divisions guarding Baghdad at below 50 percent of their normal ability to fight.

They "have been taking a pounding" for several days, Defense Secretary Donald H. Rumsfeld said at the Pentagon, adding: "They're being attacked from the air, they're pressured from the ground, and in good time they won't be there."

The Pentagon reported that the number of U.S. troops identified as killed in action or missing in action had risen to 62, with more than 100 wounded. The department's tallies, made after notification of families, often lag behind reports from the battlefield.

Commanders at Marine combat headquarters in the Iraqi desert said Tuesday's advance was intended to retake the initiative after the pause that began with debilitating sandstorms early last week and turned into an opportunity to bring men and equipment back up to standard after the swift charge of about 250 miles from the Kuwaiti border beginning March 20. While the commanders predicted a stiff fight from the beginning, Marine units moving ahead found abandoned equipment, indicating that Iraqi soldiers had retreated from their most forward positions.

In the Combat Operations Center at the headquarters of Lt. Gen. James T. Conway, the overall Marine field commander, excited officers watched real-time video images of the battlefield from a Predator reconnaissance drone. Some Marine officers have chafed at the slowdown after days of lightning progress across the Iraqi desert and were eager to be pressing the attack on the ground again. "No one likes to sit on their fingers," one Marine in the operations center said.

Fight for Kut

Kut, a trading center for fruits, vegetables and grain on the Tigris River, guards the eastern approach to Baghdad. Before heading into the capital, American planners concluded, the U.S. military had to beat down the Baghdad Division at Kut to the point where it would not be able to interfere with an attack from other fronts.

One of six Republican Guard divisions, the Baghdad has had about 11,000 men but has been augmented by a pair of tank battalions, according to U.S. intelligence. Most worrisome to U.S. war planners, however, have been the division's long-range artillery, particularly Chinese-made Type 59 guns that can fire 100mm shells nearly 16 miles and multiple rocket launchers that are capable of being loaded with chemical weapons.

"It's got a hell of a lot of artillery, and frankly that's the piece that concerned us the most and that's been the focus of much of our shaping airstrikes," Smith said.

By one estimate, according to another Marine officer, 75 percent of the Baghdad Division's artillery was hit from the air in the last few days before the first Marines began to move. Another senior Marine officer said the air campaign in recent days had wiped out half the division, leaving perhaps 6,000 men; about half of them are paramilitary irregulars without formal military training, the officer estimated.

Taking on the survivors of the bombing will be the full force of the 20,000-member 1st Marine Division. The initial move was launched at 8 a.m. local time Tuesday when part of the division began rolling up Highway 7 toward Kut. After passing through the small town of Hayy, south of Kut, the Marine force destroyed five tanks and a few Iraqi artillery batteries abandoned on the highway, according to reports reaching Marine headquarters.

Through the night, the Marines kept going, seizing key objectives and meeting little resistance. Commanders at Marine headquarters were uncertain how to interpret the sparse early enemy contact. "Either they're playing possum or maybe we really did kill them all" through air strikes, said Col. Paddy Gough, the deputy chief of operations for the Marines in Iraq.

The Iraqis did respond early this morning by launching surface-to-surface missiles against U.S. forces inside their country for the first time since the war started. One missile struck near an Army supply center around Najaf, while another pair hit northwest of Nasiriyah. No casualties were reported, and there were no indications that the missiles were fitted with chemical or biological weapons.

According to a military report, U.S. forces have located only a few of Iraq's surface-to-surface missile batteries. As of Sunday, they had not destroyed a single one of the 30 to 40 long-range Al Hussein missiles that U.S. intelligence believes Iraq has, and had struck only five to eight of Iraq's presumed 126 Al Samoud medium-range missiles and no more than 12 of 187 Ababil-100 cruise missiles, the report said.

Basra Standoff

While the battlefield heated up near Baghdad, British forces remained in a tense standoff with Iraqi troops in far southeastern Iraq around Basra, the country's second-largest city. British and U.S. forces received a request -- the first known proposal of its kind -- for a 50-minute cease-fire there to allow Iraqi troops to recover their dead. Leery of Iraqi deceptions during the past two weeks and unsure exactly who was sending the message, U.S. military commanders rejected the request.

U.S. warplanes again bombed Hussein's presidential yacht, the Mansur, near Basra in the Shatt al Arab waterway that leads into the Persian Gulf. Although U.S. planes had bombed the yacht a week ago, it did not sink. Navy officials could not confirm if the yacht sank as a result of Tuesday's airstrike.

Flares lit the night sky over Basra, although it was not immediately clear why the devices were being used. Iraqi soldiers and militiamen armed with mortars and heavy artillery still effectively control the city. Thousands of British troops have massed to the south and west, but they have been reluctant to engage in urban combat with the Iraqi forces, hoping instead that residents will rise up against government loyalists who run Basra, which has 1.3 million inhabitants.

At the same time, Army and Marine units pursued combat operations along the Euphrates River in central and southern Iraq, hunting down Iraqi soldiers and militiamen who have been attacking supply lines and ambushing rear elements of the invasion force.

In Diwaniyah, about 100 miles south of Baghdad, Marines reported killing at least 75 Iraqi soldiers and taking at least three dozen prisoners after troops on a reconnaissance mission found several fortified Iraqi positions. Intense fighting also continued in Najaf, about 90 miles south of the capital, where elements of the Army's 101st Airborne Division pressed to within a half-mile of a venerated Shiite Muslim shrine to root out several hundred militiamen believed to be hiding in the city.

Air Force planes dropped three 2,000-pound bombs on three buildings near the shrine, thought by U.S. commanders to be a resistance stronghold. But the shrine, the golden-domed Tomb of Ali, where the prophet Muhammad's son-in-law revered by Shiite Muslims is said to be buried, was reported to be unharmed.

Marines kept up pressure on regular army units to the southeast, pelting Iraq's 10th Armored Division with repeated airstrikes to keep it pinned down around the city of Amarah and prevent it from moving to help the Baghdad Division at Kut. At the same time, the 1st Marine Division, backed up by the Marines' Task Force Tarawa, moved to cut off highways that would allow in reinforcements.

Marines also swept through parts of Nasiriyah, a city along the Euphrates River about 200 miles southeast of Baghdad, to flush out militiamen who have disrupted U.S. supply lines for more than a week.

Iraq's information minister, meanwhile, read a message on state television that he said was from Hussein, calling for a jihad, or holy war, against invading U.S. and British troops.

"The aggression that the aggressors are carrying out against the stronghold of faith is an aggression on the religion, the wealth, the honor and the soul and an aggression on the land of Islam," Information Minister Mohammed Saeed Sahhaf said. "Therefore, jihad is a duty in confronting them and those who are martyred will be rewarded in heaven. Seize the opportunity, my brothers."

Earlier in the day, the Saudi foreign minister, Prince Saud Faisal, told ABC News that Hussein should step down to end the war. "Since he's asking all Iraqis to sacrifice their lives for their country, then the least that can be expected is that he would do the same and sacrifice for his country," the prince said.

The Iraqi vice president, Taha Yassin Ramadan, responded angrily at a news conference, telling Saud to "go to hell" and calling him a "failure," a "loser" and "a minion and a lackey."

If the Marines are able to defeat the Baghdad Division, that would free them to turn west and sprint toward Baghdad, whose eastern front is defended by the Republican Guard's Al Nida Division. Al Nida is considered a stronger force than the Baghdad Division and has not been targeted by U.S. bombing as heavily as other divisions yet, although parts of the unit have been cannibalized to reinforce other positions around Baghdad, U.S. officials said.

Baker reported from Marine Combat Headquarters in the southern Iraqi desert. Correspondent Alan Sipress at the U.S. Central Command's field headquarters in Doha, Qatar, and staff writers William Branigin with the U.S. 3rd Infantry Division and Jonathan Weisman in Washington contributed to this report.

The U.S. Army's 3rd Infantry Division launches missiles at the Republican Guard's Medina Division outside Karbala, where a Pentagon official said troops were in "an all-out battle."Marines of the 3rd Battalion, 11th Regiment fire on Iraqi positions with 155mm artillery near the town of Diwaniyah in south-central Iraq. Marines in the area reported killing at least 75 Iraqi soldiers after finding several fortified positions. Marines move prisoners after an engagement southeast of Baghdad. More than three dozen Iraqis were reported captured after a clash in the area of Diwaniyah, about 100 miles south of Baghdad.