We are assuring a safe and secure launch on Thursday night

Air Force Colonel Samuel Dick

The measures, which will be enforced by the US Air Force, are deemed necessary in the light of current world events.

Endeavour will also carry 6,000 US flags which will be presented to the relatives of the victims of the 11 September terror attacks.

The shuttle is set to rocket off its seaside launch pad at the Kennedy Space Center at 19.41 local time (00.41 GMT).

On board the orbiter will be Expedition Four - Ukrainian-born commander Yuri Onufrienko and US astronauts Carl Walz and Daniel Bursch - who will man the ISS until May of next year.

'Safe and secure'

American space agency (Nasa) forecasters say there is a 60% chance of acceptable weather on launch day, with Friday looking even better if rain or thunderstorms happen to delay blast-off.

But the US authorities will be taking no chances with security. No US spaceflight in history will have been accompanied by the show of force planned for launch day, sources say.

"We are assuring a safe and secure launch on Thursday night. This is an unprecedented level of effort," said US Air Force Colonel Samuel Dick, vice commander of the 45th Space Wing at nearby Patrick Air Force Base. "Whatever comes up, we'll be prepared."

Neither the military nor Nasa would say whether a credible threat had been made against the shuttle.

The skies are likely to be patrolled by fighter jets, supported by powerful plane-tracking radar on the ground.

'Positive statement'

The Federal Aviation Administration has issued new flight restrictions for launch day.

General air traffic will be prohibited within 35 miles (55 kilometres) of Kennedy's launch complex 39-B - an area six times larger than usual. Planes passing between 35 miles (56 kilometres) and 46 miles (74 kilometres) of the pad will be restricted to those with proper transponder codes and flight authorisations.

Endeavour will carry more than 6,000 US flags. When the orbiter returns from its mission, these flags will be mounted on certificates and presented to the families of the victims and survivors of the 11 September attacks.

The orbiter will also carry New York City police badges and patches, and a New York Fire Department flag and a poster showing the firefighters who died at the World Trade Center.

"It shows our resolve and the fact that we press on with our day-to-day lives and keep doing the great things that we're doing here for the space programme," mission manager Todd Corey said.

"I think that makes a pretty positive statement for the whole country."