"The enemy we're fighting is different from the one we'd war-gamed against," the army's senior ground commander in Iraq, Lieutenant General William Wallace told The Washington Post.

Over-extended supply lines combined with unconventional Iraqi tactics made a longer war look likely, he said at the Forward Operating Base Shell in Iraq in the bluntest remarks by a US commander on the war so far.

With any hopes of a quick victory dashed, the US Defense Department said Thursday it planned to more than double its troop strength in Iraq, committing another 120,000 soldiers on top of the current 90,000.

In the second week of a campaign that has seen US-led forces drive quickly to the doorstep of Baghdad but leave behind serious resistance in the south, US officers faced daunting logistical problems.

"The long distances we have travelled make it hard to push that amount of logistics -- water, fuel, ammo and chow -- over the vast area that's been covered," said marine First Lieutenant Tom Elssinger.

"It's definitely a tough animal to rope," said Elssinger, of the marines' First Regimental Combat Team.

The US Central Command, which is directing the war effort, insisted the operation was on track and pointed to the rapid progress made toward the capital in the drive to remove Iraqi President Saddam Hussein.

The US army's 3rd Infantry Division, backed by the 101st Airborne Division, was as close as 50 miles (80 kilometers) south of Baghdad, while marines advanced north in two prongs between the Euphrates and the Tigris rivers.

Waiting for them around Baghdad were at least three divisions of the Republican Guard, Iraq's best trained and most loyal forces, who were dug in defending the approaches to the city to the south, southeast and northwest.

US-led air strikes battered the Baghdad area with bombs and cruise missiles again Friday, hammering Saddam's communications sites and the Republican Guard troops guarding his capital.

Iraqi Kurd rebels also advanced Friday to within 16 kilometers (10 miles) of the northern oil capital Kirkuk in the first major movement on the northern front against Baghdad, their commanders said.

Fighters from the Patriotic Union of Kurdistan (PUK) captured the town of Qarah Anjir, situated in hills to the east of Kirkuk, after clearing scores of anti-tank and anti-personnel mines left behind by the retreating Iraqi army.

"The Iraqi army is finished. They were ordered to pull back to defend the city," Rostam Hamid Rahim, a top PUK commander, told reporters, adding that his forces were determined to seize Kirkuk, despite the risk of angering Washington and especially Ankara.

But US forces have encountered stiff resistance and atrocious weather that has significantly slowed the advance anticipated by military planners and placed a heavier burden on supplies coming up from the south.

Around the central-southern cities of Najaf, Nasiriyah and Al-Samawa, Iraq had deployed thousands of militiamen who were "fighting tenaciously," said one officer with the Third Infantry Division.

He said the drive north had been delayed by a day because troops had to be deployed around the Shiite Muslim holy city of Najaf to prevent more reinforcements from flowing in.

US President George W. Bush exuded confidence Thursday, saying alongside British Prime Minister Tony Blair that US-led forces would fight "however long it takes to win."

But two leading US dailies, the New York Times and Wall Street Journal, said Friday the war in Iraq was in trouble because of miscalculation, too much restraint and failure to assemble an overwhelming force at the outset.

"The Bush administration misread the Iraqis," the Times said in a military analysis, criticizing what it said was a war plan that counted on the Iraqis opting for an "urban-centric" defense around Baghdad.

"What the Pentagon did not understand was that the Iraqis planned to expand that strategy to include Nasiriya, Najaf, Samawa and other towns in southern Iraq," the paper said

British forces also had their hands full in the southern port of Basra, where Iraqi militia began shooting at civilians who were trying to flee in their hundreds Friday, British military spokesmen charged.

"The Black Watch are engaging them and doing their best to protect the civilians," Flight Lieutenant Peter Darling told AFP, referring to a Scottish regiment.

Darling could not say how widespread was the shooting by Iraqi forces loyal to President Saddam Hussein who have been surrounded by allied forces. There was no immediate word on casualties.

Another British spokesman Major Will McKinlay said that centres were being set up on the outskirts of the city to provide food and water to the "hundreds" of fleeing civilians who would be given safe passage.

Meanwhile the first mass shipment of humanitarian aid arrived aboard the British naval support vessel Sir Galahad, which docked Friday in the captured southern Iraqi port of Umm Qasr.

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