CNN quoted Pentagon sources as saying that elements of the 1st Marine Expeditionary Force was moving into Iraq from the southeastern border with Kuwait.

U.S. Marines have clashed with Iraqi troops and a allied bombardment has begun along the border between Iraq and Kuwait.

CNN Correspondents Art Harris and Lisa Rose Weaver reported intense attacks by U.S. artillery and aircraft on positions in southern Iraq.

New York Times reporter John Kifner, traveling with the U.S. 1st Marine Division, said Marines had engaged Iraqi troops south of the Iraqi border in Kuwait in what is believed to be the first ground combat of the war.

He added that more than 50 self-propelled Paladin cannons, with a range of 30 kilometers, were used in the barrage.

Another AFP correspondent assigned to U.S. forces near the Kuwait-Iraq border reported hearing distant artillery fire at the same time.

U.S. marine officers in the region had earlier said their units were preparing for an imminent thrust into Iraq supported by fighter-bombers, helicopters and artillery.

"It's as close to time to go as you can get," said one officer who asked not to be named.

One source said strategic targets near the Euphrates and Tigris Rivers, oil fields and military installations were expected to be the initial objectives for ground forces who would enter Iraq amid strategic bombing of the region.

Some 20,000 Marines and 5,000 vehicles from Regimental Combat Team (RCT) One and Regimental Combat Team Five would lead the push with support from the U.S. Army's third infantry division.

The marines deployed to northern Kuwait after President George W. Bush issued his 48-hour ultimatum for Iraqi President Saddam Hussein to leave Iraq, when they were gearing up for deployment to the Line of Departure.

The two Scuds and four other missiles fired on Kuwait earlier in the day forced the marines to go to an alert level known as MOPP 4 and to don their complete nuclear, biological and chemical protection suits.

At one point they were ordered to dive for cover amid fears of a pending attack.

"The Iraqis had already retaliated and sent Scuds in our direction and so we were forced to go to MOPP 4 because then we were not sure exactly what was going to happen," one officer said.

Commander of RCT One, Colonel Joe Dowdy, confirmed the attack was near and said his troops would be supported by F-18 Hornet fighter jets, Cobra helicopters, heavy artillery and Harrier jets.

He described the aerial bombardments that would precede the invasion as "shaping fire" designed to both destroy and intimidate the Iraqis into surrendering.

Overall, some 280,000 U.S. and British troops, including nearly 180,000 in Kuwait, were cocked for what American officials called a blistering "shock and awe" strike.

Baghdad was rocked by explosions barely 95 minutes after the expiration of the U.S.-imposed deadline, with more than 40 guided Tomahawk cruise missiles fired from six U.S. naval ships at what U.S. military officials called "targets of opportunity" in and around the Iraqi capital.

A senior U.S. official in Washington said the raids specifically targeted Saddam, his two sons and other senior leaders of their ruling Baath Party and Iraq's decision-making Revolutionary Command Council.

Saddam later appeared on Iraqi television unscathed, although it was unclear if the broadcast was pre-recorded.

U.S. television reports said B-2, B-1 and B-52 bombers had also joined the attack, as did two F-117 stealth fighters that dropped four satellite-guided GBU-27 "bunker-buster" bombs. There was no official confirmation.

But military planners insisted that Thursday's action, accompanied by a solemn announcement from Bush the war had begun, was not the fierce bombing blitz expected to kick off the U.S.-led offensive in earnest.

Troops from the U.S. Marine Expeditionary Unit (MEU) Fox Company 'Raiders'

Backing the U.S. and British troops were 600-700 aircraft, dozens of ships, tanks, helicopters and state-of-the-art weapons ranging from precision-guided bombs and missiles to still-secret electronic warfare devices.

The Iraqis were defended by a once-proud army that the last Gulf war in 1991 and 12 years of economic sanctions have whittled down from a million-strong fighting force to about 400,000 troops, most of them poorly-equipped and underfed.

But military experts said there remained a well-trained core that could still prove a deadly resistance to U.S. forces if, as expected, Saddam deploys them in and around his strongholds: Baghdad and his northern hometown of Tikrit.

U.S. war planners said they would open their offensive by dropping thousands of precision bombs and missiles, clearing the way for near-simultaneous land offensives and airborne assaults.

U.S. commanders say they are ready once they get the green light to push on toward Baghdad. But they in fact did not have the full complement of forces initially planned.

In a related development, Kuwaiti forces were involved in ongoing clashes Thursday with Iraqi troops on the border between the two countries, a Kuwaiti defense official said.

"Our units in the north have been shot at by small arms fire, 50-calibre (weaponry), then mortar fire," Brigadier General Abdul Rahman Mohammed al-Othman told journalists.

The Kuwaiti soldiers "returned fire," Othman said but no casualties were reported.