Military jets are set to patrol the skies, while thousands of extra police, troops and FBI agents are deployed across the United States to protect Americans celebrating Independence Day.

It is the biggest security operation ever mounted on a Fourth of July and follows a warning that terror groups might try to stage attacks to coincide with the holiday, nearly 10 months after the devastating attacks of 11 September.

The president's message to America is to celebrate, to enjoy the holiday - leave the worrying to us

White House spokesman Ari Fleischer

On the eve of Independence Day, the White House sought to reassure the American public, describing the measures as precautionary and saying there had been no specific threats.

President George W Bush's spokesman, Ari Fleischer, urged Americans to celebrate the holiday as usual.

But more than half of Americans believe an attack is likely, according to a poll published by the magazine Newsweek last week.

Variety of actions

On Tuesday, the US State Department said it had "credible" information that terrorists were planning imminent attacks against American targets around the world.

Security has also been tightened at US representations abroad

Attacks could include suicide missions and kidnappings, and US citizens and officials have been warned to be on their guard.

Terrorists could turn to soft targets - such as churches, restaurants and clubs - as security is intensified at higher-profile places, officials said.

The Homeland Security Council will monitor more than 2,000 events throughout the country, staying in touch with officials on the ground, Mr Fleischer said.

"A variety of actions are being taken on the ground in terms of greater resources, greater surveillance, greater protection, greater prevention," he added.

As well as patrolling the skies above America's major cities, forces have been deployed to protect power stations, water supplies, bridges, tunnels and airports.

The measures also include a new network of security cameras to monitor crowds in front of Capitol Hill, and special security checkpoints for those wanting to watch New York's traditional fireworks display.

Celebrating abroad

In Afghanistan, US forces are preparing to hold low-key celebrations with barbecues, volleyball and music, but no fireworks.

Security around the perimeter of Bagram air base, their Afghan headquarters, will be tightened.

In Asia, Americans also plan to celebrate the holiday with a mix of official receptions, parties and picnics, under the shadow of increased security.

"When we gather together as Americans... I am sure that this year's event will carry special meaning," Roy Tomizawa, a New York teacher in Bangkok, Thailand, told Reuters news agency.