Round-the-clock air strikes hammered this city on Saturday and yesterday as the US military sought to break the elite Republican Guard units entrenched in the sprawling city’s outskirts.

The slowdown by some units that had raced for this city early in the 11-day-old war to oust President Saddam Hussein may reflect a tactical rethink prompted by concern about stretched supply lines and stubborn, unconventional Iraqi resistance.

Failure to break into the southern city of Basra after a week’s siege may also have forced military planners to adjust.

But top US commander General Tommy Franks, who is bringing an extra 100,000 troops to the Gulf in April, insisted there was no “operational pause” in the US and British invasion.

US officers and soldiers in units in the field south of here — some are just 100 km (60 miles) away — told reporters they had orders to dig in for at least two weeks to give US airpower and artillery a chance to pound Iraqi defenses.

Franks, however, said at his command base in the Gulf state of Qatar that the war would pursue its “remarkable” progress.

He denied any pause and said the reinforcements bound for Kuwait were part of his plan.

Franks was asked if the war, which the US vice president had said would last “weeks not months”, could last into summer. “One never knows how long a war will take,” the general replied.

Fears of the damage a protracted war could inflict on the global economy have unsettled world oil and financial markets.

Saddam has vowed to make a bloody stand and inflict huge losses on the American and British invaders in street fighting.

An Iraqi military spokesman, hailing Saturday’s suicide bomb that killed four American soldiers, said 4,000 willing “martyrs” from across the Arab world were already in Baghdad to fight.

Saddam has urged Muslims to wage holy war on America and its allies — evoking one of the nightmares cited by President George W. Bush’s critics as a possible consequence of war.

Iraqi resistance in southern Shiite towns that rose against Saddam after the 1991 Gulf War has surprised the invaders who had hoped to be greeted as liberators, not shot at.

US and British officials say Saddam loyalists are using fear to deter any civilian insurgency or military mutiny. Many analysts say the Shiites are understandably wary after US forces stood aside as Saddam crushed their 1991 revolt.

British troops around Basra are calling on their experience of fighting Irish guerrillas in Belfast. A spokesman said marines fought Iraqi paramilitaries and captured a general but were still not advancing into the embattled city of 1.5 million.

Southern Iraq is still too dangerous for relief workers and, while what’s happening here may be clearer, little is known about the plight of civilians elsewhere, aid agencies say.

“The water situation in Basra is reaching a critical level,” Cassandra Nelson of the non-government Mercy Corps told reporters.

The problems British troops have faced in Basra could be a discomfiting taste of what the invaders may confront here.

In this city, unrelenting airstrikes shook the center and outskirts by day and night over the weekend.

US forces said they had targeted a training site for paramilitary Fedayeen, a presidential palace, an intelligence complex and surface-to-air missile sites here.

US forces just north of Najaf, 160 km (100 miles) south of here, shelled and mortared Iraqi troops near a bridge over the Euphrates River overnight. Near Karbala, American rockets and artillery also battered Republican Guard positions.

A soldier supervising an ammunition column to US forces further east said he was supplying front-line units with thousands of 155 mm howitzer shells, especially a version that scatters bomblets over a wide area to hit infantry.

“There is a realization that we came in a little light,” one front-line officer told his men. The officer told his men the halt may last 35-40 days, far longer than a four-six day pause mentioned on Saturday.

Soldiers dug deeper trenches and lay mines around their camp in central Iraq. “It looks like they are going to be in this position for at least two weeks,” a sergeant was heard as saying.

“They’re going to send in the aircraft to do the work before the grunts (infantry) go in,” the sergeant said.

Supply lines running 350 km (200 miles) back to Kuwait are stretched — rations are short at the front — and vulnerable to guerrillas, as Saturday’s suicide bomb showed. Even in friendly Kuwait, an attacker drove a truck at soldiers yesterday, injuring as many as 15 troops who had been lining up at a shop.

The US troops on the way to reinforce the 125,000 already in Iraq include the heavily armored 4th Infantry Division, blocked from opening a planned northern front by Turkey.