Exhausted rescue workers were last night refusing to give up hope that some victims of the World Trade Centre disaster would still be found alive.

As the official death toll for the world's worst terrorist atrocity crept towards 5,000, firefighters continued to risk their lives in a desperate search for survivors.

Earlier, hopes had been raised when five trapped firefighters were found alive. At first, it was thought they had been under the rubble since two hijacked airlines destroyed the centre on Tuesday.

But last night it was revealed that they had fallen through debris during the search yesterday.

Meanwhile, President George W Bush was preparing options for a massive military retaliation as Britain and other Nato allies indicated their readiness to back the US if any action was taken.

At least 100 Britons are believed to be among the victims of the trade centre disaster.

But Foreign Secretary Jack Straw warned that the final total of British deaths was more likely to be in the "middle hundreds".

In Washington, Congress was working on legislation authorising military retaliation, while the president himself was reported to be considering sending special forces into action if any country could be identified as having helped the extremists.

Last night, Secretary of State Colin Powell named Osama bin Laden as a prime suspect.

However, the Taliban regime in Afghanistan, where he is hiding, continued to insist on his innocence.

The vast search to uncover the terrorist plot stretched from Miami to Boston, to Portland, Maine, and on to Canada and Germany.

Yesterday afternoon, a man in his mid-40s was arrested under the Prevention of Terrorism Act at Heathrow Airport. Police would not say whether the arrest was linked to the US attacks.

Up to 50 terrorists are believed to have been involved in the operation, with at least four hijackers trained at flying schools in the US.

Last night, the black box from a hijacked plane which crashed in Pennsylvania was found, triggering hopes it would provide vital clues.

The FBI believe the plane was going to be used to attack Camp David, but there have been claims the White House may have been the target.

Mr Straw said that British forces were in a state of readiness for any response to the attacks, but did not wish to discuss what action might be taken.

Japan and Australia also rallied behind battle plans, while China's president phoned President Bush and offered his nation's help in rescue efforts, and stronger cooperation against terrorism.

As the official death toll rose, survivors told grim tales of their escape from the World Trade Centre towers as bodies rained from the sky "like rag dolls".

Irishman Ronnie Clifford, 47, said he was on the ground floor when there was an explosion and a badly-burned woman staggered out of the haze.

"It just looked like she was totally charred," said Mr Clifford. "She came from where I had just walked and she came through the revolving doors with her two arms totally burned, her clothes were burned right on her body.

"She didn't know what was happening or where she was going. I was screaming for emergency help."

Mr Clifford, who in a terrible twist lost his sister Ruth, 44, and niece Juliana, four, who were on one of the doomed jets, added: "I went back out on to the street and you could see people falling from the building like rag dolls, jumping from the building because they had nowhere to go."

It also emerged that workers in the second tower were told it was safe to stay at their desks after the first jet slammed into the neighbouring building.

After two days of confusion, the number of dead became clearer when New York's mayor said 4,763 people had been reported missing in the city.

Combined with the preliminary death toll of 190 in the Pentagon and the 45 killed in the Pennsylvania crash jet, the estimate so far of those killed is running at 5,000.

The toll compares with the 2,390 Americans who died at Pearl Harbour and the 1,500 dead on the Titanic.

There was an unconfirmed report yesterday that the body of a suspected hijacker and an air stewardess with her hands tied had been discovered in the wreckage.

The New York landscape remained a haze of grey dust, some of it asbestos, twisted girders and boulders of broken concrete.

In London, solidarity came from hundreds of people who queued up to sign five books of condolence at the US embassy, in scenes reminiscent of the aftermath of the death of Diana, Princess of Wales.

A special Changing of the Guard ceremony was held at Buckingham Palace before thousands of people, among them many US tourists who were moved to tears when a band played their national anthem.

There were also moves towards getting back to normal as some flights resumed to and from the US.