WASHINGTON -- U.S. and Italian officials were warned in July that Islamic terrorists might attempt to kill President Bush and other leaders by crashing an airliner into the Genoa summit of industrialized nations, officials said Wednesday.

Italian officials took the reports seriously enough to prompt extraordinary precautions during the July summit of the Group of 8 nations, including closing the airspace over Genoa and stationing antiaircraft guns at the city's airport.

But a U.S. official said that American counter-terrorism experts considered the warning "unsubstantiated."

In either case, the reports suggest that Western governments were aware that terrorists might one day use a hijacked airplane as a suicide weapon--as they did Sept. 11 in attacks on the World Trade Center and the Pentagon.

The Genoa warning was disclosed last week by Italian Deputy Prime Minister Gianfranco Fini. In remarks on a television talk show reported by the Italian news agency ANSA, Fini said: "Many people were ironic about the Italian secret services. But in fact they got the information that there was the possibility of an attack against the U.S. president using an airliner. That's why we closed the airspace and installed the missiles. Those who made cracks should now think a little."

An attack on the summit would have endangered not only President Bush, but also British Prime Minister Tony Blair, French President Jacques Chirac, Russian President Vladimir V. Putin and others.

In an interview published Sept. 21 in the French newspaper Le Figaro, Egyptian President Hosni Mubarak said his government provided information to the United States about possible attacks on the Genoa summit by Saudi-born terrorist Osama bin Laden. "There was a question of an airplane stuffed with explosives. As a result, precautions were taken."

White House aides refused to comment on the reports. "We just don't talk about security arrangements," spokeswoman Anna Perez said.

But a U.S. official outside the White House said the Genoa reports were received and discounted.

"There were some press reports citing what we subsequently determined was unsubstantiated information," said the official, who spoke on condition of anonymity.

In any case, the possibility of suicide hijackings has been known to U.S. counter-terrorism officials for several years.

On Christmas Eve 1994, Algerian terrorists hijacked an Air France Airbus and planned to blow it up over the Eiffel Tower in Paris. French troops stormed the plane as it was refueling in Marseilles and killed the hijackers.

The hijackers' organization, the Armed Islamic Group, is now believed to be part of Bin Laden's Al Qaeda network.

In 1996, a terrorist captured in Manila told Philippine police that Al Qaeda planned to hijack 11 U.S. airliners simultaneously and to fly a plane into CIA headquarters near Washington.