By Lesley Curwen
BBC News, Washington
US President George Bush has reiterated his staunch defence of a controversial electronic eavesdropping programme.
The White House is on an offensive to justify the eavesdropping
He made the remarks during a visit to the US surveillance headquarters, the National Security Agency.
It is the most recent in a series of robust speeches by the administration to justify surveillance of Americans suspected of links to terrorists.
Critics argue that the programme is illegal and interferes with civil liberties.
This is the latest salvo in a battle by the Bush administration to justify secret eavesdropping on Americans' phone calls and e-mails abroad without getting the usual permission from courts to do so.
Under an order issued by the president soon after the 11 September attacks in 2001, the National Security Agency no longer needed a special permit to do this.
Democrats in Congress and some Republicans have expressed concerns that this is illegal and constitutes a gross abuse of presidential power and an infringement of civil liberties.
Duty to protect
Mr Bush's speech centred on arguments that his first duty was to protect the US people and that authorities must be able to quickly detect when someone linked to al-Qaeda is communicating with someone inside the US.
The president cited the recently-broadcast taped message said to be from Osama Bin Laden as evidence of the continuing threat to the US and he issued a challenge to his critics.
"All I would ask them to do is listen to the words of Osama Bin Laden and take them seriously. When he says he's going to hurt the American people again, or try to, he means it," he said.
"I take it seriously and the people of NSA take it seriously, and most of the American people take it seriously as well," he told them.
This speech was part of an all-out assault on public opinion.
Earlier in the week, Alberto Gonzales, who heads the Department of Justice, argued that the US Congress had given the president authority to order the surveillance programme.
The attorney-general has said the spying is necessary
And on Monday, General Michael Hayden, one of the country's most senior intelligence officers, appeared in public to defend the legality of the programme.
The campaign by the administration addresses opinion polls which suggest a majority of US citizens favour an investigation into whether the law has been broken.
The US Senate is due to start hearings on the surveillance programme on 6 February.