No stopping Sept 11: Rice

US national security adviser Condoleezza Rice defended the Bush administration's counter-terrorism action before the September 11 attacks and insisted "no silver bullet" could have prevented the devastating strikes.

Rice said in sworn testimony to the official inquiry into the 2001 attacks that left about 3,000 dead that President George W Bush understood the threat from al-Qaeda as soon as he took office and that it took priority over Iraq.

Bush's trusted adviser highlighted a longstanding US failure to understand the terrorism had been consistently inadequate for years before Osama bin Laden's group struck.

"For more than 20 years, the terrorist threat gathered, and America's response across several administrations of both parties was insufficient," she said.

"Tragically, for all the language of war spoken before September 11, this country simply was not on a war footing," she said.

The security adviser, who had for weeks resisted giving open testimony to the panel, insisted that the Bush administration had taken al-Qaeda seriously.

"President Bush understood the threat, and he understood its importance. He made clear to us that he did not want to respond to al-Qaeda one attack at a time.

"He told me he was "tired of swatting flies", Rice said in her opening statement.

Defeating al-Qaeda was Bush's first security directive after he took office in January 2001, she said. "It was the very first major national security policy directive of the Bush administration - not Russia, not missile defence, not Iraq, but the elimination of al-Qaeda."

But she insisted that the United States had no intelligence that could have prevented the attacks.

"The threat reporting that we received in the spring and summer of 2001 was not specific as to time, nor place, nor manner of attack," she said.

"There was no silver bullet that could have prevented the 9/11 attacks," Rice said.

Rice's statement contradicted earlier testimony by the White House's former counter-terrorism czar Richard Clarke, who told the panel Bush had failed to consider the al-Qaeda network an urgent threat until the attacks.

Clarke also accused Bush of diverting to Iraq attention and resources that were needed in Afghanistan to hunt down al-Qaeda leader Osama bin Laden.

But Rice said none of Bush's advisers called for an attack on Iraq in the days after September 11.