He said the level of resistance witnessed by British forces in the south during the first week of war, especially around Iraq’s second city of Basra, showed the US-led campaign to seize control of Baghdad would be hard fought.

Pressure would instead continue from the skies — he called Thursday night’s US-led bombing the war’s heaviest — as part of a “hit hard and wait” campaign aimed at pressuring Iraqi President Saddam Hussein into defeat.

“The key thing is that US forces do not want to get involved in downtown fighting. It’s all about applying pressure with the result of the regime falling,” the source told Reuters on condition of anonymity.

“Basra is very interesting to watch as a precursor, an example of how Baghdad will be tough to crack,” he said.

British officials said yesterday that Iraqis had fired mortars and machine-guns at about 1,000 civilians fleeing Basra as control of the city continued to evade ground forces.

Iraq said 116 people had been killed and 695 people wounded in Iraq’s second city since war began on March 20, with repeated reports of local uprising and fleeing Iraqis failing to translate into total control.

“It does represent the scale of resistance,” said the source, amid growing expectations war may prove longer and tougher than many pundits had first predicted.

President George W. Bush, after talks with top ally British Prime Minister Tony Blair on Thursday, made a clear attempt to extinguish expectations for swift victory, saying the war would be won — “no matter how long it takes”.

Military sources told Reuters that predictions US-led troops could surround Baghdad in five to 10 days were “entirely plausible” and within the timetable. “It could be quicker,” said one British official.

“There is a natural operational pause that could just take a couple of days. You cannot commit to an operation and then go hell for leather all the time,” the official said.

But with British forces focused on southern Iraq, the Americans to the north lack the punch to win a street fight in the capital, stronghold for Iraq’s elite Republican Guard, said the defense source.

“You need an overwhelming force to move into urban areas and I don’t think we will do it,” he said. “We do not have the amount of troops you need to carry out urban guerrilla warfare.”

The United States on Thursday ordered 120,000 more troops to the Gulf and a US general said fierce resistance and guerrilla tactics pointed to a longer conflict than planners had forecast.

Officials said the extra 120,000 US soldiers would be in Iraq by the end of April, doubling the size of its force, in what they called a long-developed plan.

Officials say there are no current plans for Britain to augment the 45,000 troops it has already committed to the war.

“We offered up to the Americans the number of troops that we felt were necessary to complete the tasks that we’ve been given and that is it,” said the source. “For the foreseeable future, we don’t plan to commit any further troops.”