Prime Minister Tony Blair declared last night: "We are at war with terrorism" - and told the world Britain would not flinch from military strikes against those responsible for the attacks on America.

"This is a time for a calm head and cool nerves. We will see this thing through. It will be done," Mr Blair pledged from the Cabinet Room at 10 Downing Street, in an interview shown around the globe.

He predicted grimly that the British death toll could rise to between 200 and 300, making Tuesday's suicide hijackings the worst terrorist outrage committed against UK citizens.

As tension increased, attention focused on Afghanistan - base of the suicide hijackings' "prime suspect" Osama bin Laden - as the likely first target of what the Pentagon announced would be called Operation Noble Eagle.

Foreign aid workers and civilians yesterday fled the capital Kabul, and the ruling Taliban regime there said it had fortified military bases and airfields in readiness for attacks.

Britons were ordered by the Foreign Office to evacuate border areas of neighbouring Pakistan.

A statement from bin Laden repeated his denial of involvement in Tuesday's strikes on the World Trade Centre and the Pentagon.

But Britain said it would gather its own evidence before judging whether he was behind the suicide hijackings.

A delegation of senior Pakistani officials will go to Afghanistan today to demand that bin Laden is handed over to the US.The decision was made yesterday amid countrywide demonstrations by hard-line Muslims, who oppose Pakistan's decision to give its "full support" to the US.

US Secretary of State Colin Powell said America had to be prepared for further attacks, which could even involve "chemical, biological or radiological" threats.

Security was increased even further around President Bush amid fears of further strikes, as the total death toll from Tuesday's attacks topped 5,500.

The hunt for survivors continued, but New York Mayor Rudolph Giuliani admitted hope was fading for the chances of anyone else being found alive.

Mr Blair's staunch backing for military strikes was delivered in an interview for the American CNN network.

A sombre Mr Blair was asked whether Britain, too, was now at war. He replied without hesitation: "Yes. Whatever the technical or legal issues about a declaration of war, the fact is we are at war with terrorism.

"What happened on Tuesday was an attack not just on the United States, but an attack on the civilised world."

The Government has made special arrangements to fly British relatives of victims to New York, where trained bereavement counsellors will be waiting at the British consulate to assist them.

Mr Blair said: "There has to be a response to bring those terrorists who committed this attack to account and we will play our full part in that.

"Secondly, there has to be an agenda we construct at an international level to dismantle the machinery of international terrorism."

The Prime Minister said he would not back away from supporting military action. "We have made it clear that we stand side by side with the United States," he said.

"The US is in close consultation with us and with other allies and I think there will be very broad support for a response which allows us to pursue and bring to account those responsible and do it as an act of justice."

Mr Blair added: "When we merely offer words of support, that's the easy part in one sense. The hard part starts when we actually take the action."

But it was "too premature" to talk of what the specific military response would be.Later, speaking outside No 10 as he left with his family to pray in nearby Westminster Cathedral, Mr Blair said: "Are we at war with the people who have committed this terrible atrocity? Absolutely.

"We have to assemble the evidence, present it and then pursue those responsible." He added: "This is a situation where we have got to be prepared to act and we will act - in a calm and a measured way - but act we must."

Mr Blair's steely rhetoric came after spending the weekend at No 10 in talks with senior ministers, military and intelligence chiefs and in phone conversations with other leaders.

Meanwhile, the investigation into the four hijackings by 19 terrorists was being stepped up, with the FBI issuing an arrest warrant for a second unnamed man it considers "a material witness".

Dozens of people have been questioned by investigators and an unknown number remain in custody, held for alleged breaches of immigration rules.

One "material witness" is also being held, along with two men, Ayub Ali Khan and Mohammed Jaweed Azmath, who were arrested on a train to Texas with no legal identification and with craft knives like the ones used by the hijackers.

Last night, President Bush predicted America would win decisively what he called "the first war of the 21st century".

Speaking after he touched down at the White House after a weekend at Camp David, he said those who had attacked America would be destroyed.

"I want to remind the American people that the prime suspect's organisation is in lots of countries," he said. "This will be a long campaign, a determined campaign, a campaign that we will use the resources of the United States to win."