More than 30,000 American reinforcements were ordered to the Gulf Wednesday night as fierce battles raged through southern Iraq and the Republican Guard went on the offensive.

U.S. paratroopers were also dropped into northern Iraq, where they seized a key airfield in the first clear sign that America was opening up a northern front. The deployments came as war planners were forced to change tactics and put the battle for Baghdad on hold. Pentagon chiefs conceded that they had underestimated the resistance they would face.

The push towards the capital has been severely hampered by repeated attacks on armored columns and supply convoys trying to bolster the American front line 50 miles south of Baghdad. These ambushes have fuelled criticism that the Pentagon went to war with too few troops.

Now one of America's most modern fighting units, the 16,000-strong 4th Infantry Division is being sent to Kuwait, where its 200 tanks, other vehicles and equipment are waiting. It will be joined by 14,000 more troops from other units, including the 3rd Armored Cavalry. The 4th Infantry should have been used to open the northern front through Turkey, but the plans were blocked by Ankara. Instead, 1,000 paratroopers from the 173rd Airborne Brigade were the first substantial force to move into the north Wednesday to pave the way for a new wave of American troops and firepower.

Officials said that several hundred special forces troops had been operating in the area and that supplies and equipment for the airborne brigade would follow, although it was not clear how they would be delivered. The reinforcements were ordered in after a series of setbacks in the opening days of the war.

U.S. forces were involved in a major battle Wednesday with Iraqi units for control of a bridge over the Euphrates, 13 miles southeast of Najaf. A number of U.S. Abrams tanks and Bradley fighting vehicles were destroyed by Iraqi rocket-propelled grenades.

Further south, an 80-vehicle U.S. Marines supply convoy came under heavy attack after it had stopped just north of Nasiriyah, another strategic crossing point.

The Marines could also encounter the Republican Guard earlier than expected: A convoy of hundreds of vehicles and thousands of crack troops moved out of Baghdad yesterday under cover of sandstorms, heading straight towards the Marines deployed in Nasiriyah. U.S. intelligence said that Republican Guards had been spotted in two towns along Highway 7 and were being attacked by warplanes.

Gen. Richard Myers, chairman of the Joint Chiefs of Staff, said that they were being engaged "where we find them." Another smaller column of up to 100 Iraqi tanks was heading out of Basra toward the al-Faw peninsula. This group came also under fierce attack from coalition warplanes and tanks. Bad weather further hampered the allied offensive, grounding fleets of U.S. helicopters for a second day and preventing jets from taking off from aircraft carriers.

With coalition forces failing to achieve the strides north that they had hoped for or to take towns in the south, the Pentagon has had to shift tactics. U.S. officials said that they would have to focus their attention on dealing with the resistance in the south before they could put everything into the march north. The Pentagon war plan had envisaged a lightning march on Baghdad, buoyed by popular support in the newly liberated south. The aim had been to provide a solid platform for the swift toppling of President Saddam Hussein, possibly from within his inner circle.

But American officials now concede that they had underestimated the guerrilla-style resistance led by the 40,000-strong Fedayin militia that is loyal to Saddam and the vulnerability of the American supply line. One official said, "We didn't think the Fedayin would play such a big role. It's not a huge deal, but we will have to adjust accordingly."

Others said that rather than keep pushing north, the allied forces would have to confront the enemy in the south before amassing troops and firepower near Baghdad. However, that means losing the momentum that was supposed to be the key to success.

President Bush continued to try to rally U.S. morale as he prepared to meet Tony Blair for a summit at Camp David, insisting that American forces would be "relentless in our pursuit of victory." U.S. forces were well prepared for the battles ahead, he said. "I can assure the long-suffering people of Iraq that there will be a day of reckoning for the Iraqi regime, and that day is drawing near."

He accepted, however, that the war was "far from over" and backed away from declaring that the U.S. was "ahead of schedule," a phrase White House officials had said he would use. American officials and the vice president, Dick Cheney, had suggested that success in the war would be quick, but yesterday Bush sought to play down expectations while insisting that the result was not in question. "We have an effective plan of battle and the flexibility to meet every challenge," he said.

Blair arrived in Washington yesterday evening after a flight in which his chartered Boeing 777 was struck by lightning. The aircraft was jolted and there was a loud bang similar to the sound of under-carriage coming down as a white flash ran alongside, but the plane landed safely at Andrews Airforce Base 20 minutes later. The prime minister flew from there directly to Camp David for talks with Bush. The war council was focusing on plans for the administration of a post-war Iraq, with Blair pressing for the deep involvement of the U.N.