British troops of 16 Air Assault Brigade fight through the smoke to secure oil fields

The advance from Kuwait

The hastily erected handwritten sign at the exit to the UN demilitarised zone read "Welcome to Iraq from the fighting 61st. Have a nice day!"

As British troops advanced north yesterday to try and prevent an environmental disaster in one of the world's largest oil fields, their first sight was the small wooden board left by the US Marines.

Throughout the Rumeila oil fields, towers of black smoke and searing flames could be seen emanating from the small well-heads dotted across the landscape around the gas and oil separation plants (Gosps) in a reminder of Saddam Hussein's torching of Kuwaiti oil fields following the last Gulf conflict.

This time, however, coalition forces were determined to avert destruction on a similar scale to that of 12 years ago. While US Marines were sent in to seize the plants west of Basra, the dangerous task of removing explosives left by the Iraqi army fell to the British 16 Air Assault Brigade.

As they advanced from Kuwait, at least nine well-heads were already on fire, ignited by retreating Iraqis. Fierce orange to bright yellow flames belched forth in ferocious mushroom formations before forming thick, acrid spirals of smoke. Thousands of brigade vehicles formed a convoy, dozens of miles long, snaking 60kms into southern Iraq, after Royal Engineers cleared fields of anti-tank and anti-personnel mines.

Scattered across the sand was evidence of an earlier battle fought by the US Marine 5 Regiment Combat Team, which reportedly left two Iraqi soldiers dead and several injured. The twisted metal wreckage and ammunition of the past two days' conflict mingled with the rusty remains of the first Gulf War and dummy tanks placed as decoys. Above, Gazelle and Huey helicopters circled as they carried out reconnaissance missions.

Elsewhere Iraqi armoured vehicles, in a square formation with their guns pointing north, were evidence of those who had chosen to follow coalition guidelines – transmitted through Psyops leaflets and radio broadcasts – on how to surrender.

Nearby soldiers stood guard on sand ramparts surrounding a newly built prisoner of war camp. Almost a hundred inmates could be seen sitting below, eating from what appeared to be US army ration packs.

The few locals along the road yesterday waved and smiled. A group of young men, speeding past in a pick-up truck, their headdresses blowing in the wind, greeted the invading forces with glee. Dromedary camels with loping gaits appeared oblivious to the passing vehicles while wild dogs barked in vain.

Small camps of remaining Marines with armed lookouts posted could be seen along the road, as infantry from the Royal Irish Regiment began setting up camps around the Gosps. Their job was to secure the fields, as experts were brought in to fight the flames, protected by the brigade's elite helicopter 3 Regiment Army Air Corps.

Lynx Mk7s will circle to the north and east, tasked with picking off any incoming Iraqis with their anti-tank missiles while the Gazelles carry out reconnaissance to order in the back-up of fast jets if necessary. The regiment was also using one of their Lynx Mk9s to transport commanding officers and in case of any need to evacuate downed pilots.

With well-heads already aflame, there were fears the rest were rigged for demolition as Royal Engineers went in to search for mines and explosives placed around the well-heads. One US Marine officer was killed on Friday and several injured by mines peppered around the oil fields, and clearing them proved a painstaking process for British experts.

Their task was made even more precarious by burning trenches of oil set alight in an attempt to hamper soldiers and disorient the pilots.

British troops, co-ordinating with the Americans as part of the Marine Expeditionary Force, were posted throughout a 1,800sqkm area into Iraq, with the Household Cavalry and the 3 Para regiment fortifying the northern edge as the US Marines moved on.

"Our objective is to secure the infrastructure, prevent what destruction we can and an ecological disaster. The oil fields are key to rebuilding the country and we have got to save them for the Iraqi people," explained Lieutenant Colonel George Butler, commanding officer of the 3 Regiment Army Air Corps.

The Rumeila fields, which provide a large proportion of Iraq's vast reserves, are primarily manned by 3,000 migrant workers from the Indian subcontinent, who were told to leave their posts at the beginning of the week.

Leaflet drops and radio transmissions had been used in the previous weeks to try and convince the workers they were safe to return to their posts in the hope of getting the wells back to normal production as fast as possible. "Black gold" is Iraq's greatest asset and the key to any prosperous future. The country has proven reserves of 112 billion barrels, making it second only to Saudi Arabia. Yet under UN sanctions it produced just 2.8 million barrels a day. Oil analysts estimate that figure could be doubled and a US-controlled government would lead to deals for privatised production with US and British firms.

Larry Goldstein, president of the Petroleum Industry Research Foundation, said recently that while the war might not be about oil "the day the war ends it has everything to do with oil".

It had been a classic game of "hurry up and wait" for the 3 Regiment Army Air Corps and its fellow troops from the 16 Air Assault Brigade. Having left its base at Camp Eagle in Kuwait early on Thursday, the brigade gathered in the south before making its way through the desert in a blacked-out convoy to a second destination. Early on Friday morning the brigade made its way across a landscape so barren that only wreckage broke up the skyline, to a point just 11km south of the border.

As they waited to advance, there were countless calls to take cover in sand trenches as the Iraqis fired in retaliation. While coalition Patriot missiles disabled half of the 10 missiles, several exploded without any casualties. Tense soldiers put out a series of false alarms, hitting their horns or shouting "gas, gas, gas" and donning respirator masks.

Despite having some of the most sophisticated gas detection equipment in the world, the sound of bombardment sparked a panic among soldiers who immediately followed the protective procedures drummed into them over the previous weeks.