More than half of all Americans believe their country is likely to be attacked again during the national 4 July holiday, according to an opinion poll.

The findings came as the Federal Bureau of Investigation (FBI) issued a warning about potential terrorism using fuel tankers, and follows other scares involving allegations of overheard phone calls and suspicious behaviour.

The Bush administration has frequently warned Americans that their country is in grave danger of further attacks.

The message has obviously hit home, as 57% of people surveyed in a Time magazine/CNN poll said they believed an attack on the Independence Day holiday was likely.

A party on Brooklyn Bridge was cancelled due to terrorist threats

US intelligence agencies have been heavily criticised for failing to act on information received before the attacks on 11 September, and over the past months they have issued a number of warnings about possible new attacks.

The FBI continues "to receive information from detainees and other sources about possible terrorist activities," said spokesman Bill Carter.

Most of the information is uncorroborated or unconfirmed, Carter warned, "but out of an abundance of caution, we provide this information to law enforcement agencies".

Fuel tanker warning

On Friday, the FBI issued a warning that fuel tanker trucks could be used in attacks against American targets in the United States or abroad.

Bureau spokesman Paul Bresson said the organisation had received uncorroborated information that the targets could be fuel depots, Jewish schools or synagogues.

The FBI has also spent time investigating warnings that terrorists are planning an attack on Las Vegas, after a man said he intercepted a phone conversation in Arabic last week in which such an attack was discussed.

A scare in Newark, New Jersey, has meanwhile been resolved. A man of Middle Eastern appearance who tried to buy an ambulance has been traced and found to have a perfectly innocent reason for wanting the vehicle.

Rumours have also been circulating in Washington about a possible attack on the city's subway system and even the possible use of scuba divers to launch an attack.

In May, a birthday party for New York's Brooklyn Bridge had to be cancelled after warnings of possible attacks on American landmarks.