Officials See Plot As Worst Threat Since 9/11

The White House framed the apparently thwarted terrorist plot as a direct attack on the US yesterday, as America responded to the news from London with a nationwide tightening of airport security.

With officials describing the plot as the greatest terrorist threat to the US since September 11 2001, George Bush called the arrests a "stark reminder" that the country was "at war with Islamic fascists who will use any means to destroy those of us who love freedom, to hurt our nation". Speaking in Wisconsin, the president said: "It is a mistake to believe there is no threat to the United States of America."

Delays grew at airports after homeland security officials banned all liquids and gels from aircraft cabins except for baby formula and medicines. The official terror threat level was raised throughout the aviation system, requiring airlines to provide US authorities in advance with the name of every passenger travelling from Britain.

Federal security officials were focusing on John F Kennedy airport in New York, Dulles airport outside Washington, and Los Angeles International airport as the probable destinations of the planes involved, they told state-level officials privately. A spokesman described officers at the US Northern Command in Colorado as "a little bit more vigilant" than normal.

By the middle of yesterday, additional armed air marshals had left the US for London, where they were to join US-bound flights, mingling unidentified with passengers, the homeland security department said. One of the carriers reportedly targeted in the plot, American Airlines, cancelled several flights to London, blaming delays at Heathrow.

US officials emphasised a suspected link with al-Qaida. The homeland security secretary, Michael Chertoff, called the disrupted plans "suggestive of an al-Qaida plot", while Robert Mueller, the FBI director, said the scheme "had the earmarks of an al-Qaida plot". A senior counter-terrorism official told the Associated Press that up to 50 people might have been connected to the conspiracy.

Alberto Gonzales, the attorney general, suggested there might be prosecutions in the US, though there had been no arrests or evidence of plotting on American soil.

By Wednesday, Mr Chertoff said, the potential attackers "had accumulated and assembled the capabilities they needed, and were in the final stages of planning for execution ... this is not a case where they were just in the initial thought stage".

Repeatedly citing the British legal system as a reason for withholding further information, officials painted a picture of close collaboration between British and American investigators that had been stepped up in the last two weeks. Mr Chertoff said that was when Washington had received the first definitive clues that the plot would target American planes specifically. But there was still uncertainty, he said, "about whether the British have scooped up everybody".

A former senior homeland security adviser to Mr Bush told the Guardian that the US ban on liquids would probably be temporary. "Can we have an entirely foolproof system? Probably, but at what cost, psychologically or economically?" said Frank Cilluffo, director of the homeland security policy institute at George Washington University in the US capital.

The official US terror threat level was raised to red, or severe, its highest level, for commercial flights originating in the UK and bound for the US, and to orange, or high, for the aviation system. The general nationwide threat level remained unchanged at yellow, or elevated.

Showing a markedly more relaxed attitude than British authorities, Mr Chertoff said US passengers should "go about their plans confidently, while maintaining vigilance in their surroundings, and exercising patience with screening and security officials".