British and American troops were involved in fierce fighting near Iraq's main port today as the war to topple Saddam Hussein began.

The firefight broke out near Basra as men of the Special Boat Service targeted the strategically vital city and the oilfields in southern Iraq.

At the same time allied troops were flooding into the demilitarised zone on the Iraqi border with Kuwait 40 miles away to take up positions for an all-out invasion.

Cruise missiles were also loaded onto B52 bombers at RAF Fairford in Gloucestershire, a clear sign that the bombardment of Baghdad could be only hours away.

British troops taking up "forward battle positions" were ordered to switch off satellite phones and allied warplanes bombed targets in Iraq after coming under fire in the no-fly zone.

By lunchtime, allied forces were in position to strike from the moment the 48-hour deadline set by President Bush for Saddam to quit Iraq expires at 1am British time tomorrow. But the White House had refused to rule out a strike before that.

The fighting reported at Basra was believed to involve British special forces and US marines in an operation to prepare landing sites for amphibious craft during an invasion.

Other special units were deep inside Iraq on secret operations to prepare landing strips in the desert for airborne troops.

Basra, Iraq's only seaport, lies on the Shatt al Arab waterway where the Tigris and the Euphrates open into the northern Gulf.

Surrounded by treacherous sandbanks and marshes it is difficult to approach from the sea.

Artillery, infantry and the tanks of the 7th Armoured Brigade had already moved into Forming Up Positions, and some were already on the start line.

An attack could target Basra and proceed up alongside the Euphrates towards the strategic cities of Nasariya, Najaf and Karbala.

Tony Blair said he believed all MPs, irrespective of their views on the war, now wished British troops well.

"I know everyone in this House wishes our Armed Forces well," he said in the Commons.

A sandstorm whipped across northern Kuwait as the pace of preparations suddenly quickened Kuwaiti security sources disclosed that allied troops move into the demilitarised zone, which straddles the Iraq-Kuwait border, at around 11am local time, 8am UK time.

The source, working in the Umm Qasr area in the east of the zone, said: "American convoys are still driving towards Umm Qasr."

A US military spokesman said he could not confirm or deny that troops were inside the zone.

A British Army spokesman said only that soldiers had taken up " forward battle positions".

At Fairford, 14 giant American B52 bombers which will lead the fight against Saddam were loaded up with cruise missiles this morning.

The first flight of B52s were expected to take off two hours before sunset to give them enough flying time to identify their targets and drop their first devastating payload before heading for home.

The missiles were driven to the aircraft in five articulated lorries escorted by police at 10.30am.

Troops meticulously loaded the weapons - each costing around £1million - into the bomb bays by forklift truck.

With an estimated flight time of only six hours to Iraq the bombers are expected to play a huge part in the initial air bombardment. A single B52 can deliver a payload of more than 70,000lb at a range of 8,800 miles without being refuelled. They are likely to take up positions over the Mediterranean or the Red Sea to unleash cruise missiles or satelliteguided smart bombs. RAF Tornados, Harriers and Jaguars are also likely to be involved in the opening 48-hour offensive.

The Tornados will be given the specific task of taking out air defences and barracks round small missile batteries and air strips in the Iraqi desert.

This will enable the enemy positions to be quickly seized by airborne forces and turned into bases for the advancing allied armies.

The Harrier force of up to 20 planes has the job of supporting special forces, the SAS and Special Boat Service and American Rangers in the hunt for Scud missile sites and any artillery shells with chemical warheads. Intelligence suggests Saddam has given his generals personal authority to unleash the deadly weapons as a last desperate measure to hold the Allies off from attacking Baghdad.

The mainstay of the bombing attack will be the 750 American and British fighter bombers from Gulf bases and the six American aircraft carriers now at battle stations in the Mediterranean and the Arabian Sea.

The aircraft, including RAF Tornados and Harriers, F16s, F15s and F18 Hornets will work on a "taxi rank" basis, forming ranks in the air before being sent in on targets. Along with the B52s from Fairford, other longrange bombers include the almost mythical B2 Spirit bat-wing supersonic aircraft which will fly from bases on Diego Garcia. Also spearheading the attack will be B1B Lancer and F117 Stealth bombers.

Action began in the air today as warplanes from the USS Abraham Lincoln bombed Iraqi positions after coalition aircraft - including two RAF Harrier jets - were fired on by Iraqi forces.

"There were, yesterday, four firings against our aircraft flying in the southern no-fly zone," Rear Admiral John Kelly told reporters on board the Lincoln. He said US forces had responded by bombing "a series of targets" he described as "command and control" positions.