Secret Service officers cleared staff, reporters and school tours out of the building and two F-16 fighter jets raced to the spot after radar blip appeared to show a plane had intruded the restricted airspace around Washington.

When the jets arrived in the area of the supposed violation they could find no aircraft, said Lieutenant Colonel Rob Garza, spokesman for North American Aerospace Defence Command (NORAD) said.

Asked whether the incident was the result of a false radar reading possibly triggered by atmospheric conditions or birds, Garza replied "it could be."

"The F-16s scrambled in response to an air space violater, but when the aircraft got out there they couldn't find anything," said Garza.

Violated

The alert triggered an evacuation of White House staff at just after 9.20am (1420 GMT).

"There was never a plane. It was a blip on one radar"

Rebecca Trexler,
Federal Aviation Authority

"There was a report that airspace had been violated, standard precautions were taken," said Trent Duffy, a White House spokesman.

"It was all over in a matter of minutes."

A US aviation official confirmed that the skies had remained safe.

"There was never a plane. It was a blip on one radar," said the Federal Aviation Authority's Rebecca Trexler.

Crossed wires led a senior administration official to tell AFP that a plane had violated restricted airspace but was "contacted" by air traffic controllers on the ground.

High alert

The president is currently on a state visit to London where he faces widespread public protests over the US-led war on Iraq, which he has repeatedly justified with reference to the September 11, 2001 plane attacks on US landmarks.

On November 10, a similar security alert prompted the US Secret Service to evacuate Vice President Dick Cheney after a tourist plane violated the aerial security zone.

The plane was intercepted by air force jets and escorted out of the area.