NEW YORK/WASHINGTON — In the worst attack on American soil since Pearl Harbour, three hijacked planes slammed into the Pentagon and New York’s landmark World Trade Center yesterday, demolishing the two 110-storey towers that symbolise US financial might. The attacks brought normal life across the United States to a standstill, turning the major cities of the nation into eerie ghost towns. All financial markets were closed, millions of workers sent home early, all flights around the nation were cancelled and all airports shut in an unprecedented move.

Key landmarks like the White House, the fire-damaged Pentagon, the Sears Tower in Chicago, the CIA building in Washington and the Walt Disney theme parks on both coasts were ordered evacuated and closed until further notice.

No death toll was immediately released but officials feared the number of victims could climb into the thousands — possibly the tens of thousands — as 40,000 people alone worked in the soaring steel and glass Trade Center towers.

It was the worst attack on American soil since Japanese war planes bombed the US fleet at Pearl Harbour on December 7, 1941, killing 2,280 soldiers and 68 civilians and forcing the United States into World War II. Former secretary of state Henry Kissinger told CNN, "This is comparable to Pearl Harbour and we must have the same response and the people who did it must have the same end as the people who attacked Pearl Harbour."

There were a total of 266 people on board the day’s four hijacked planes — two that crashed into the twin towers, one that slammed into the Pentagon and a fourth that crashed in a wooded area near Pittsburgh, Pennsylvania.

"I wouldn’t want to say what the death toll could be. It will be a horrible number. I saw people dropping out of windows," a shaken New York City Mayor Rudolph Giuliani told ABC News, adding that 200 firefighters were missing.

"I looked outside and saw a big chunk of the World Trade Center missing," said Verizon employee Ellen Leon. "Fifteen minutes later I saw people jumping out of the building. Bodies were flying out.

I don’t know if they were already dead or if they were just going to die." No group took immediate responsibility for the attack but suspicions centred on an implacable US enemy — Osama bin Laden, whose followers were held responsible for murderous attacks on US embassies in Africa.

Yesterday’s attacks triggered scenes of panic, disbelief and heroism in the nation’s largest city, where police and firefighters risked their lives to save people from the 110-storey twin towers before its 200,000 tonnes of steel frame came smashing down to the ground covering lower Manhattan in a snow storm of soot.

"It’s nuts, there is debris and dust everywhere, and it looks as though a volcano erupted down there," said Michael DeVita, a euro-dollar trader.

DeVita was working on the 84th Floor of World Trade Center Building No. 2 when the first plane hit building No. 1. DeVita said once the first plane hit, people were immediately evacuated from building No. 2.

Hospitals in New York were overwhelmed with patients as a black cloud billowed into the blue skies over Manhattan where the city skyline had been dramatically and permanently altered.

"Hundreds of people are burned from head to toe," said Dr Steven Stern at St. Vincent’s Hospital in the Greenwich Village neighbourhood of lower Manhattan. Rescue workers used commuter ferries to carry victims across the Hudson River to safety in Hoboken, New Jersey, where the scene resembled a war zone, with victims laid out on stretchers, limping on crutches, and others walking without a shirt and with their pants torn.

Dozens looked shellshocked and were weeping in nearby streets. The thick plume of smoke rising from what used to be the World Trade Center was clearly visible in the background.

Bridges and tunnels between New York and New Jersey were closed, making it impossible for parents to return home and pick up their children from school.

President George W. Bush vowed to bring those responsible for the attacks to justice as he stopped in Louisiana to talk to the nation. "Make no mistake, the United States will hunt down and punish those responsible for these cowardly acts," the president said.

Speaking at the Emma E. Booker Elementary School, before the full horror had unfolded, Bush said: "Terrorism against our nation will not stand. Today we’ve had a national tragedy."

He called for a moment of silence. "May God bless the victims, their families and America," Bush said, his voice breaking with emotion.

He went on to Nebraska for a national security briefing.

"We have not seen an attack like this, certainly not since Pearl Harbour," said Adm. Robert Natter, commander of the US Atlantic Fleet, which was dispatching ships and aircraft for air defence, along with amphibious troops, to Washington and possibly New York.

The twin towers collapsed in a huge cloud of smoke and fire, about two hours after the initial impacts. Desperate people were seen jumping out of the burning towers before they collapsed.

Throngs of people raced up lower Broadway blocks north of the terror attack, yelling "It’s coming for us," "Run," and "We’re going to die," with smoke wedged between the skyscrapers of the financial district.

"I’m scared and I want to go home," said Lloyd Clark, a stunned bus driver who was standing on a corner waiting for his handicapped passengers to be evacuated from a nearby building.

"What are the odds of two planes hitting the same building?" said one New York City police officer.

American Airlines said two of its jets carrying a total of 156 people were lost. United Airlines said two of its planes had crashed, one in Pennsylvania, with a loss of 45 lives in that disaster.

As international flights were diverted to Canada, the Federal Aviation Administration shut down all flights in the United States.

World leaders expressed shock and horror and foreign financial markets fell sharply on news of the attacks. The London FTSE index plummeted 5.7 per cent, while oil prices spiked up. US markets were closed.

The day of horror began just before 9am (1300GMT) in New York, when the first plane plowed into the south tower of the World Trade Center, as thousands of workers were streaming into the building to begin their day.

It opened a huge hole near the top of the building. TV stations caught the second plane plowing into the second of the twin towers, exploding in a fire ball a few minutes after the first impact.

The two towers caved in shortly after the impacts, one after the other.

A third plane crashed into or near the Pentagon outside Washington, throwing people off their feet inside the building and setting off a massive fire. Later still, a fourth plane crashed — this time in a wooded near Pittsburgh. A total of 266 people were reported on board the four planes.

Bin Laden believed to be in exile in Afghanistan, was blamed for the 1998 bombings of the US Embassies in Kenya and Tanzania in which 224 people died.

An Arab journalist with access to Bin Laden said in London the renegade Saudi had warned three weeks ago of an "unprecedented attack" on US interests.

Washington has offered a $5 million reward for his capture. George Tenet, director of the CIA, said Bin Laden was the most immediate and serious threat to US security.

Beside the embassy bombings, US officials link Bin Laden to last year’s bombing of a US Navy ship in Yemen and with foiled plots in the United States and Jordan at the turn of the millennium.

The previous worst act of terrorism in the United States was the 1995 bombing of the Alfred P. Murrah federal building in Oklahoma City in which 168 people died. Timothy McVeigh was executed for that attack earlier this year

The earlier bombing of the World Trade Center in 1993 resulted in six deaths and hundreds of injuries.