US troops captured Baghdad airport last night as coalition forces edged closer to the gates of the Iraqi capital.

Tanks and armoured units met almost no opposition from Iraqi forces as they seized Saddam Hussein international airport.

ABC television reporter Bob Schmidt, travelling with the 3rd Infantry Division, said from the airport: "US forces encountered very little resistance, although some units did encounter scattered firing by Iraqi foot soldiers and men in pick-ups."

He also reported seeing Iraqis waving and cheering as US tanks rolled towards the airport, which is only ten miles from central Baghdad.

Sky News correspondent Colin Brazierm said: "The Americans didn't think there was a possibility that the airport would simply be surrendered in the way it appears to have been."

But according to the CNN TV network, civilians in Baghdad were being ordered by Iraqi authorities to drive to the airport to defend it,

The network said that all Iraqi checkpoints at the entrances to Baghdad had been closed for the first time.

The coalition's advance units were only four miles from the edge of Baghdad late last night, following a fast-moving two pronged advance on the city.

A string of explosions rumbled through the capital, which was dark with power off for the first time since the bombardments began.

It threatened to disrupt water supplies and the city's sewage system, which could mean widespread disease at a time when temperatures are rising.

On the approach to Baghdad itself, scores of burnt-out Iraqi army vehicles and dead fighters littered the roadside.

Piles of green uniforms had also been discarded, suggesting many Iraqi soldiers had sought to blend in with civilians.

Two divisions of the Republican Guard - the Baghdad and Medina - have been destroyed, but four more were reported to be moving towards the advancing Coalition Forces.

Thousands of US military vehicles moved across the Euphrates River after taking a bridge at Musayyib, 35 miles from Baghdad, which had been rigged with explosives by the Iraqis.

Marines had moved across the Tigris and up Highway 6 towards Baghdad.

In the town of Kut, on the Tigris, the marines fought building to building with Iraqi forces.

Air Marshal Brian Burridge, commander of British forces in the Gulf, said the advance on Baghdad was the most impressive military manoeuvre he had seen.

"They will be writing about it in staff colleges for decades," he said.

About 50 miles from Baghdad, US special forces launched a dramatic helicopter raid on Saddam Hussein's Tharthar presidential palace.

Guards returned fire but US troops got into the palace and found documents belonging to the Iraqi regime.

With US forces charging forward and meeting little resistance, there were fears elements of the Republican Guard had retreated, trying to draw the Allied forces into bloody street battles.

Brigadier General Vince Brooks played down suggestions of an immediate invasion of the capital. He said: "We are having success now but we believe there is fighting ahead and we can't predict exactly how that will unfold."

US forces may follow the British example at Basra of making occasional raids and waiting for the regime to "implode". British forces spokesman Group Captain Al Lockwood said: "We hope that ultimately the regime will realise how ridiculous it is to continue with this effort and surrender en masse."

However, military commanders warned that Saddam's fanatical Fedayeen fighters could carry on their war for years, becoming "sleepers" and launching an on-going terrorist-style war.

Iraqi Information Minister Mohammed Saeed al-Sahaf said the Allies would be defeated in a long war. "They've not been able to control any Iraqi city," he told reporters in Baghdad. "We're waging a war of attrition against this snake and we will be victorious."

Saddam was shown laughing in a television broadcast.

Near Karbala, 50 miles south of Baghdad, a Black Hawk helicopter was downed by an Iraqi surface-to-air missile killing six, officials in Washington said.

A Navy F/A-18C Hornet was also downed and the US said it appeared to have been hit by a Patriot in a "friendly fire" incident.

They were also reports of another "friendly fire" incident involving an F-15E Strike Eagle in which one soldier died.