The United States has refused to blame Al-Qaeda for a plot to explode US-bound airliners, which Britain claimed it had thwarted last week.
"It looks like something Al-Qaeda would do," White House spokesman Tony Snow said on Tuesday, speaking to reporters after President George W Bush visited the US National Counter-Terrorism Center (NCTC) in McLean, Virginia, a suburb of Washington.
"But our intelligence, at this point, does not permit us to say with confidence that that was the case," Snow said.
Snow acknowledged there have been public accusations of Al-Qaeda for the plot, which Britain said on Thursday it had foiled, and reports of foreign governments blaming the terrorist network.
"Before we actually claim Al-Qaeda, we want to make sure that we ... Could prove it to you," Snow said.
On Monday Bush himself had cautiously echoed the remarks of his Homeland Security secretary, Michael Chertoff, who last week said the plot bore the hallmarks of Al-Qaeda.
"It looks like the kind of thing Al-Qaeda would do," Bush said after working meetings on national security.
But, he added: "I'm not going to say it until we have absolute proof.