Osama bin Laden has admitted for the first time responsibility for the attacks on the World Trade Center and Pentagon.
In a chilling video which has been circulating among his followers for 14 days, a smiling bin Laden says the twin towers were a "legitimate target" and the suicide pilots were "blessed by Allah".
"History should be a witness that we are terrorists. Yes, we kill their innocents," he says.
In the tape obtained by Britain's Sunday Telegraph and shot in the mountains of Afghanistan at the end of last month, he justifies his deeds, saying: "There are two types of terror, good and bad. What we are practising is good terror. We will not stop killing them and whoever supports them."
He also directly threatens British Prime Minister Tony Blair and warns nations such as Australia and Japan to stay out of the conflict.
Mr Blair has ordered British embassies in the Muslim world to publicise the video, the release of which follows threats by bin Laden to use chemical and nuclear weapons.
While Washington said there was no credible evidence of a specific nuclear threat, President George W. Bush acknowledged publicly that bin Laden's al Qaeda group was seeking nuclear, chemical and biological weapons.
The New York Times later revealed that two sites had been identified in Afghanistan where deadly chemicals were suspected, yet these had not been attacked in the month of bombing by US forces.
One, a fertiliser plant near the town of Mazar-e-Sharif, was captured by Northern Alliance soldiers on Saturday. The other was a research laboratory in a small village in eastern Afghanistan.
Adding to the concern, a newspaper in Peshawar, Pakistan, claimed yesterday that authorities there and in the US were investigating if al Qaeda could have already transported nuclear, biological and chemical weapons into America.
The Frontier Post said Pakistan's ISI, the FBI and CIA fear two briefcase nuclear weapons could have been acquired by al Qaeda from rogue Central Asian groups. The newspaper said one was Russian made and contained 2kg of fissionable plutonium and uranium. It gave its serial number and manufacturing date of October 1988.
American experts did not rule out such a possibility. "The probability is not zero," said Tim Brown, an intelligence and military analyst with GlobalSecurity.org. "It's somewhere between zero and low."
Analysts who have examined the threat said one possibility could be a so-called "suitcase nuke", probably from the ex-Soviet Union, being sold to terrorists, who would seek to smuggle it into the US, or within range of an US overseas interest.
During the Cold War, the US and Soviet Union each produced a few hundred portable nuclear weapons.
The Frontier Post also said 70 capsules of a Russian-made biological agent were thought to be in al Qaeda hands.
Bin Laden made his nuclear threat in an interview with a representative of one of Pakistan's biggest newspapers in a mud hut near the Afghan capital, Kabul.
He said he would unleash nuclear or chemical weapons if the US used similar weapons against him.
"We have the weapons as a deterrent," the Dawn newspaper quoted bin Laden as saying.
US intelligence experts doubted the nuclear weapons claim but believe his fighters have experimented with crude chemical weapons at a training camp in Afghanistan.
"They're seeking chemical, biological and nuclear weapons," Bush said in Washington. "Given the means, our enemies would be a threat to every nation and, eventually, to civilisation itself."
In other developments in the War Against Terror:
* In the most dramatic defeat for the Taleban since bombing began, the Northern Alliance not only stormed into an abandoned Mazar-e-Sharif but took control of three provinces in the north and west.
* Pakistan's President Pervez Musharraf has ordered his nuclear arsenal redeployed to at least six secret locations to prevent attack or capture by hostile elements, the Washington Post reported.
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Links: War against terrorism
Timeline: Major events since the Sept 11 attacks
Bin Laden boasts 'We did it' in chilling video
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