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

[Agencies should] make contact with appropriate Jewish community representatives and trucking and fuel delivery facilities... and emphasize the need to report suspicious activity or persons

FBI

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 it has issued a number of warnings about possible new attacks.

The FBI said local police forces should emphasise the need to report suspicious behaviour in their communities

But in New York, Jewish communities shrugged off the warning.

"As of my experience, I don't think it is a threat," said Mendel Rosenberg, a member of an unarmed citizen's group, the Shomrin Patrol, that monitors Jewish areas in Brooklyn.

Series of alerts

The fuel truck warning is the latest in a long series of security alerts to be issued by the FBI.

Two days ago, it warned of possible terrorist attacks at 4 July celebrations.

The warning said there was no specific information that an attack was planned but said terror groups could target significant dates.

Other FBI warnings include 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.

Leaks investigated

The warning came as leaders of a congressional panel investigating the 11 September attacks asked the Justice Department to investigate leaks from their hearings.

The president has deep concerns about the inappropriate disclosure of information that can compromise sources.

White House spokesman Ari Fleischer

On Wednesday, anonymous congressional sources told reporters that the National Security Agency (NSA) had intercepted two messages on 10 September that could possibly have referred to the attacks.

Messages allegedly received by the NSA on 10 September - in Arabic - read: "Tomorrow is zero hour" and "The match is about to begin". But they were not translated until after the attacks, according to sources quoted by the Washington Post.

The White House has complained about the disclosure of the messages, saying it could be harmful to America's efforts to prevent further attacks.

National Security Agency

One of 13 US federal intelligence agencies Based in Fort Meade, Maryland Director: Lieutenant General Michael Hayden Protects information sent through US Government equipment Intercepts and deciphers foreign signals Budget and staff numbers are secret

Senator Bob Graham and Representative Porter Goss, who chair the joint congressional committee, asked the Justice Department to investigate who, among congressmen and their staffs, leaked the information.

The Justice Department said it would "expeditiously review this matter and take any appropriate action".

"The president has deep concerns about the inappropriate disclosure of information that can compromise sources and methods and potentially interfere (with) and harm America's capacity to fight the war against terror," White House spokesman Ari Fleischer said.

On Tuesday the two messages were put before the committee, which is looking into events leading up to the attack and how intelligence agencies can avert future attacks.

Members of the panel are questioning senior FBI, CIA and NSA officials behind closed doors.

Lieutenant General Michael Hayden, the NSA director, was reportedly asked by the committee why it took his agency two days to translate the messages.