Days after it appeared that the battle had been won in Iraq's south, the shooting -- and the dying -- goes on.

At Basra, allied forces -- circling Iraq's second largest city, refusing to enter it for fear of being caught in an urban bloodbath -- came under heavy artillery fire Monday. A British soldier was reported killed in action nearby.

At the southern oil fields, once considered secure, shadowy Iraqi forces apparently ambushed a British unit by feigning surrender. Civilian workers brought in to fight fires at the wells were forced to withdraw for their own safety.

At the port of Umm Qasr, continuing battles choke the lifeline of humanitarian shipments for Iraq's suffering people.

The resistance is sporadic and sometimes fierce, generally the work of irregular Iraqi forces like the Fedayeen, Saddam Hussein's most trusted paramilitary fighters. They wear no uniforms and hide among civilians, striking and then receding.

"It felt great when we came in, with the crowds waiting and smiling. Now you wonder what's behind those smiles -- and what lies behind those crowds," said Lt. Col. Michael Belcher of the 1st Marine Expeditionary Force. "It's tough to win over their hearts and minds now, when you have to hold them at arm's length."

But official military line was that this was all to be expected.

"This is not a videogame, where everything is clear and neat and tidy," said Lt. Col. Ronnie McCourt, a spokesman for British forces in the Persian Gulf. "Some enemy who feel that they want to carry on fighting will inevitably do so. We have contingency plans for this."

In Basra, those plans include encircling the city in a kind of loose siege. Civilians are allowed to come and go, after they are searched for weapons.

"Basra is surrounded and cannot be used as an Iraqi base," British Prime Minister Tony Blair told the House of Commons. "But in Basra, there are pockets of Saddam's most fiercely loyal security services who are holding out. They are contained, but still able to inflict casualties on our forces, so we are proceeding with caution."

A reporter for the newspaper The Scotsman, attached to British troops on the front line outside of Basra, said Iraqi mortar rounds rained down through the day, with tanks and armored fighting vehicles of the Black Watch and the Royal Scots Guards returning fire.

U.N. Secretary-General Kofi Annan said "urgent measures" needed to be taken to restore the Basra's electricity and water supply and prevent a humanitarian disaster. Much of the water supply has been out since Friday.

But even if Basra was secured, the flow of water and other humanitarian aid has been impeded by the continuing battle for Umm Qasr, the port city at the very tip of the Gulf.

Gen. Tommy Franks, the U.S. war commander, said ships "will begin to deliver needed humanitarian assistance, food, water, medicine, to Iraqis within the next few days." But Lt. Cmdr. Steve Tatham of the Royal Navy in Bahrain said no such ship would go to Umm Qasr until the town was secured.

Farther up the Euphrates, the Marines reported Sunday that they had taken control of the Iraqi Navy port of Az Zubayr, near Basra. But on Monday, artillery and mortars opened fire after a Marine patrol reported being fired on.

It was near Az Zubayr that the British soldier was killed Sunday night while trying to calm rioting Iraqi civilians, The Daily Telegraph newspaper reported.

In the area of the Rumailah oil fields, armed Iraqis in civilian clothes -- some of them possibly using woman and children as screens -- were operating, said U.S. military officials.

Coalition forces seized the oil fields in the opening hours of the ground war to prevent the Iraqis from destroying the wells or dumping oil into the Gulf. Civilian firefighters were brought in to put out the fires that were raging when the allies arrived, but pulled out after an incident Monday.

"Yesterday, we captured five POWs that just drove up, waving a white flag. They just surrendered to us," said Brian Krause, vice president and senior blowout specialist for the Houston-based Boots and Coots.

"A little while later, five more POWs drove up to some British soldiers waving a white flag, and when they got close they opened up with machine guns."

British officials could not confirm the incident or any casualties. But U.S. Marines declared the oil fields unsafe for journalists to visit, forcing the cancellation of a trip under Marine escort intended to give the media a firsthand view of the blazing wells.

"It's not nearly as safe as they said it was," Krause said. "We're kind of sitting ducks out there."