AMERICAN troops were digging deep trenches south of Baghdad last night as Pentagon chiefs signalled that they would not advance on the city for weeks.

Plans to assault the Iraqi capital have been postponed until reinforcements arrive on the US front line and coalition bombers have tried to wear down Republican Guard units.

America’s top military officer, General Richard Myers, sought to make the most of the delay, saying: “The one thing that we have on our side is patience. With the outcome never in doubt, we can afford to take our time and set the conditions on the battlefield. We’re going to bring everything to bear there.”

Washington was anxious not to present the tactical shift as a setback, even though US forces have failed to secure their 250-mile supply line in 11 days of fighting. Donald Rumsfeld, the US Defence Secretary, insisted that there were “no plans for pauses or ceasefires”, a message echoed by General Tommy Franks, the US commander in the Gulf.

But their comments related to the overall military effort, rather than the advance on Baghdad. Coalition bombers sustained their intense aerial campaign over the city, hitting a presidential compound, an intelligence complex and air defence batteries.

The artillery bombardment of Republican Guard positions around Karbala, some 30 miles south of Baghdad, also intensified. But plans for ground forces to push on to President Saddam Hussein’s seat of power have been put on hold. “We are only going to start a major push at a time of our advantage,” General Myers said.

The decision marks a victory for the military over their civilian masters, who have been pressing the political imperative for a swift end to the war. President Bush encouraged his generals to keep their sights fixed on Baghdad, although he is leaving military planning to General Franks.

But stretched and vulnerable supply lines have left frontline American troops short of food, water and fuel. And the message from them was markedly different from the reassurance of US Central Command that everything was on track.

The troops have now been told that there will be no attempt to advance for at least two weeks. One officer told his men the delay could last for 45 days. “There is a realisation that we came in a little light,” he is reported to have said. Troops were digging deeper trenches and foxholes, laying mines around their camp, and camouflaging vehicles.

The ground assault may wait until the ground force is almost doubled with the deployment of an extra 100,000 US troops. Some 30,000 of them belong to the high-tech 4th Infantry Division, which was supposed to open a northern front into Iraq from Turkey under the Pentagon’s preferred war plan. They were rerouted to Kuwait after Ankara blocked the plan. Other reinforcements are unlikely to arrive before the end of April.

Ministers in Britain and Australia also signalled that the advance would have to wait. Mike O’Brien, the Foreign Office minister, said that military forces in Iraq were “re-shaping”, and the Australian Defence Minister said the US had “underestimated” Iraq’s irregular units.

Tariq Aziz, Iraq’s Deputy Prime Minister, said the war was going very well for his country. He was surprised to hear that American troops expected to be welcomed with flowers. “The Iraqi people are going to receive the Americans with bullets,” he said.

And coalition chiefs were forced to confront a new danger after four American soldiers were killed by a suicide bomber. General Hazem al-Rawi, Iraq’s military spokesman, said that 4,000 Arab volunteers had arrived to carry out more such attacks. “Martyrdom operations will continue not only by Iraqis, but by thousands of Arabs who came to Baghdad,” he said. “They left their countries and families to come here and seek heaven.”

The political dangers of a protracted military conflict were meanwhile bought home to Tony Blair when his former Cabinet colleague Robin Cook wrote an article saying that he had had enough “of this bloody and unnecessary war”, which was supposed to have been quick and easy. “I want our troops home and I want them home before more of them are killed,” he wrote. But Cabinet ministers condemned the former Foreign Secretary, and David Blunkett said that Mr Cook had squandered the dignity of his resignation.

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