Democrats and some Republicans question the legality of Bush's decision, taken after the September 11 attacks, to authorize eavesdropping without court approval if one of the parties on the call was suspected of links to terrorism.

Former Vice President Al Gore, the Democrat who lost the 2000 presidential election to Bush, has called for a special counsel to investigate whether the president broke the law, and Congress is to hold hearings on the issue.

The 1978 Foreign Intelligence Surveillance Act makes it illegal to spy on US citizens inside the United States without approval from the special FISA court.

Bush said he acted to allow the National Security Agency to move more quickly in monitoring suspect communications to head off possible further attacks.

He said a measure passed by Congress after the September 11 attacks gave him authority to use whatever force was necessary against the enemy in his declared "war against terrorism."

Bush dismissed critics who say he should have worked with Congress to change the 1978 law, passed to halt intrusive domestic spying under President Richard Nixon.

"My concern has always been that, in an attempt to try to pass a law on something that's already legal, we'll show the enemy what we're doing," Bush said at a news conference.

He said the spying programme was "so sensitive and so important" that if information was revealed about how it operated, it would help the enemy.

"And so, of course, we'll listen to ideas. But I want to make sure that people understand that if the attempt to write law makes this programme -- is likely to expose the nature of the programme, I'll resist it," Bush said.

According to a new New York Times/CBS News Poll, more than half of Americans approve of the White House's domestic eavesdropping programme to fight terrorism, but nearly two-thirds worry that Americans' civil liberties could be threatened by US antiterrorism programmes.

Sen. Dianne Feinstein, a California Democrat, said, "I have been deeply troubled by the fact that the administration, if it believed it needed additional authorities related to domestic electronic surveillance, did not seek legislation granting them that authority."

The White House has been on a public campaign to reassure Americans that this is a limited programme and the government is not engaged in a mass spying operation.

In that effort, the White House says the programme should not be referred to as "domestic spying" because one end of the link being monitored is overseas. The White House is calling it "the terrorist surveillance programme."

Bush said Gen. Michael Hayden, the former director of the National Security Agency, came forward with the programme as a tool for fighting terrorism after the September 11 attack.