WASHINGTON: US President George W. Bush on Thursday signed a law expanding legal authority for wiretaps by spy agencies which he hailed as vital for America's security, after a fierce Congress battle.

"This law will play a critical role in helping to prevent another attack on our soil," Bush said as he signed the bill, calling it "vital to the security of our people."

"The bill will allow our intelligence professionals to quickly and effectively monitor the communications of terrorists abroad while respecting the liberties of Americans here at home," he said.

The law includes retroactive immunity for telecommunications firms which aided warrantless government surveillance operations following the September 11 attacks in 2001, meeting a key White House demand.

The bill had sparked fierce debate between civil liberties advocates who argue it eroded checks on the power of government and intelligence officials who feared the row was compromising their power to thwart terror attacks.

Senators finally voted on Wednesday 69 to 28 to pass the measure, after blocking several attempts to water down the immunity for telecom firms.

"Almost seven years have passed since that September morning when nearly 3,000 men, women, and children were murdered in our midst," Bush said.

"The attack changed our country forever. We realised America was a nation at war. Against a ruthless and persistent enemy."

The president added: "The most important lessons learned after 9/11 was that America's intelligence professionals lack some of the tools they needed to monitor the communications of terrorists abroad.

"It's essential that our intelligence committee know who our enemies are talking to, what they're saying, and what they're planning."

After the September 11 attacks, Bush authorised the National Security Agency to eavesdrop on calls and emails between the United States and abroad in cases that federal agents deemed may have a terror link.

The wiretaps went ahead without the permission of a special court set up to watch over government wiretapping operations inside the United States, provided for under the Foreign Intelligence Surveillance Act (FISA) of 1978.

The programme, revealed in 2005, caused public outcry and opponents argued that US privacy guarantees meant the intelligence agencies should seek court warrants from the FISA court to conduct such spying inside the country.

The new law says the FISA court mechanism is the only way the government can order electronic surveillance of terror suspects.

The 1978 law allowed the ultra-secretive National Security Agency to wiretap for 72 hours while waiting for the FISA court to approve the action.

The new law, however, gives the agency a week, and it also allows them to use any information they got even if the FISA court eventually rules that the wiretap is unlawful. - AFP/de