The United States will hunt down and punish those responsible for these cowardly acts

President Bush

Thousands are feared to have died inside the buildings when hijacked airliners slammed into them. About 260 more are presumed to have perished on board the planes.

Another building close to the World Trade Center, Salomon Brothers 7, collapsed several hours after the original catastrophe.

New York remains in a state of intense fear and confusion, made worse by a thick fog of ash, fumes and debris from the ruins of the Trade Center.

No-one has said they were responsible for the attack, but US officials have pointed the finger at Saudi dissident Osama bin Laden, an Islamic militant.

In a series of spectacular blows:

• Two hijacked passenger airliners plunge into the twin towers of the World Trade Center, the subsequent explosions and fires causing the buildings to collapse

  People fled screaming from the World Trade Center

• The Pentagon is partly destroyed by another hijacked plane which crashes into it shortly after the New York attacks, setting it ablaze

• Unexplained explosions are reported near the State Department and Congress buildings in Washington

The explosions caused the twin towers to collapse

The plane's intended target was believed to be the Maryland presidential retreat, Camp David.

President Bush, who cut short a visit to Florida on hearing the news, said the US would hunt down and punish those responsible.

Click here to see graphic of New York attacks

Mr Bush is now returning to Washington, where he is expected to address the nation on Tuesday evening.

The following security measures have been taken:

• All government buildings in the country were evacuated, including the White House and Congress

• Lower Manhattan was evacuated and the National Guard brought in

• All commercial flights to and from the US were suspended until noon on Wednesday (1600 GMT)

• Air force fighters were ordered to attack any suspicious aircraft flying in US airspace

• Seven US warships sailed towards New York, two aircraft-carriers to help in rescue efforts

He gave no specific figure, but said 1,500 walking wounded had been evacuated to a nearby park, and another 600 were being treated in local hospitals.

Dr Steven Stern at St Vincent's Hospital in the Greenwich Village area of lower Manhattan said: "Hundreds of people are burned from head to toe."

Appeals have been issued for supplies of blood to help treat the injured.

Pentagon collapse

In Washington, the Pentagon - the heart of the US military establishment - suffered a direct hit from another hijacked passenger jet.

The Pentagon is the heart of America's defence machine

Part of the five-sided structure collapsed and secondary explosions were reported as huge clouds of smoke rose from the wreckage.

Pentagon spokesman Glenn Flood said there were "extensive casualties and an unknown number of fatalities".

United Airlines confirmed that two of its planes had crashed with a total of 110 people on board - one in Pennsylvania and the other in New York.

American Airlines said it owned the other two planes lost, which were carrying 156 passengers altogether.

World leaders have reacted with shock and outrage to news of the devastating attacks.

Among those sending messages of condolence were President Vladimir Putin of Russia, UK Prime Minister Tony Blair and French Prime Minister Lionel Jospin.

However, some Palestinians and other Arabs have been celebrating the attack on the United States - seen by them as Israel's main sponsor.

People screaming

One of the eyewitnesses who e-mailed BBC News Online said: "I came out of the subway, just after the first plane hit to scenes of absolute chaos. People were running away screaming.

I heard the bomb and saw both buildings crumble like biscuits

Eyewitness

"The sight of a hole in the side of the building was completely shocking. The neighbourhood is covered in a thick pall of smoke now the second tower has collapsed."

The initial impact of the collapse on the ground was devastating.

"It was like the scene out of a movie as the huge ball of rubble grew behind a terrorised crowd, running for cover," said BBC News Online's David Schepp.

Associated Press reporter Dunstan Prial said he heard a sucking sound just before the first building collapsed.

"Windows shattered. People were screaming and diving for cover. People walked around like ghosts, covered in dirt, weeping and wandering dazed," he said.

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