The city centre and the west bank of the Shatt al-Arab waterway were still under Iraqi control, the correspondent confirmed, and said the British incursion extended only as far as the western and southern suburbs of Mechrank and Andalus.

Around midday local time (0800 GMT) shots were fired from the central area at an unmanned reconnaissance plane but failed to hit the target.

And in the early afternoon, warplanes flew low over the city for the first time but no bombs were dropped, the correspondent added.

Earlier British news agency reports, quoting military officials, had claimed the tanks had reached the centre of the city, but a spokesman for the Irish Guards who were involved in the incursion, said they were still some two kilometres from the centre.

Back at the Guards' base, some four to five kilometres from the centre, Captain Michael Garraway said: "Our battle group pushed about 2.5 km from here they took out a number of positions defended by the Fedayeen and other militia groups.

"We're not sure of the number of casualties, there are surely not any among us, and a number of Fedayeen were destroyed."

He added: "We're not physically occupying the positions, it's not our business to go in and occupy the houses, we'll try to hand the city back to the people who own it."

A British spokesman at the war headquarters in Doha said their forces were intent on seizing more territory as a show of force for local residents and militias in Iraq's second city, which has been encircled by British-American troops for about two weeks.

Group Captain Al Lockwood told the BBC that several armoured brigades had moved forward.

"Our initial objective is to take some of the outskirts of Basra, set up vehicle checkpoints ... and to take some ground," Lockwood said.

The advance, Lockwood later told CNN, was intended "to reassure the people of Basra that we are there, to show the para-militaries that we are there and that we are controlling the area that we are now in."

Earlier, British military spokesman Chris Vernon said in Kuwait that the decision to send in the tanks was made after battle planners estimated that the Iraqi's "defences have been weakening."

The so-called Desert Rats, from Britain's 7th Armoured Brigade, met "patchy" resistance as the troops moved in from the southwest, an unnamed military source told the Press Association, Britain's domestic news agency.

The source, at Central Command in Qatar, said there was some "stiff fighting" early Sunday morning in the city, with a population of some 1.5 million people.

Royal Marine Commandos were said to have launched a second major offensive on the southwestern outskirts of Basra, according to unsourced Press Association reports.

The elite forces began the attack with heavy shelling at about 3.30 pm (1230 GMT) as they edged ever closer to Iraq's second city, the news agency said.

burs/ps/dab

SPACE.WIRE