Bush said that although the US was safer than before the September 11 attacks, it would be a mistake to believe there was no longer a threat.

Speaking on a visit to Green Bay in Wisconsin on Thursday, Bush said the foiled plane plot was "..a stark reminder that this nation is at war with Islamic fascists who will use any means to destroy those of us who love freedom, to hurt our nation".

"The American people need to know we live in a dangerous world, but our government will do everything we can to protect our people from those dangers," he said.

The US government heightened security on commercial flights and forbid passengers from taking liquids on board with their hand luggage.

American officials said the aim was to blow up up to 10 planes in flight.

Two officials said there were no signs the attacks were directed at any one city, but they might have taken place on flights heading to major US cities.

Pakistan "helped UK"

US Homeland Security Secretary Michael Chertoff said the plot "had a lot of members, and it was international in scope."

He said the operation was to involve "multiple explosions in multiple aircraft."

Meanwhile a Pakistani official said his country's intelligence service helped British security agencies foil the alleged plot.

"Pakistan actively cooperated to make this happen," the official said. "There was very close cooperation between the intelligence agencies of the two countries."

The official said action was being taken in Pakistan, but would not confirm whether any arrests had been made in the country.

Earlier the French interior minister, Nicolas Sarkozy, said the suspected perpetrators appeared to be of Pakistani origin.