TOKYO - Japan is studying a contingency plan that would allow its air force to shoot down hijacked commercial aircraft during the Group of Eight summit in July, government sources said on Thursday.

Japanese officials are working on detailed plans to protect leaders of the G8 industrial nations, which are due to meet on the northern island of Hokkaido on July 7-9, the sources said.

Under one of the plans being considered, F-15 jet fighters from Japan's air force would scramble to force hijacked aircraft to land at a nearby airport.

"But if that attempt fails and the plane is thought to be heading towards the G8 site to attack the summit, we might have to shoot it down," one of the sources said. "That would be the worst-case scenario we are considering."

Another source, however, expects security to be so tight that any hijacking will be unlikely. "In any case, we have to be prepared," the source said.

He said the area in and around the G8 summit venue would be declared a "no fly zone" and armed police officers would be aboard all commercial planes in Japan.

Defence Minister Shigeru Ishiba declined to comment on whether Japan was considering shooting down hijacked planes.

"Of course we have to consider all possible situations. But we cannot talk about what we are considering," he told reporters.

The Sankei newspaper reported earlier this month Japan may position missile interceptors around the site of the G8 summit.

Japan is used to handling protests at similar summits.

In 1986, a Japanese leftist group fired home-made missiles over the heads of Group of Seven leaders, including U.S. president Ronald Reagan, at the welcoming ceremony in central Tokyo.