An anonymous bomb threat e-mailed to Air China's Tokyo offices on Friday forced a passenger jet to make an emergency return to Japan, the Japanese Transport Ministry said. Four other flights were delayed.

The e-mail, written in Japanese and received in the early afternoon, urged the airlines to suspend its flights or the writer would "bomb the aircraft," said Transport Ministry official Fumio Yasukawa.

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"We suspect this is a threat related to the Olympics," he said, refusing to provide further details of the note or say whether any particular groups were suspected of sending it.

A flight carrying 70 people from Nagoya to Shanghai was forced to return to Japan after the threat was received, and it landed safely, he said.

Two other jets underwent safety checks and departed Fukuoka, in southern Japan, one headed to Beijing and one to Shanghai, Yasukawa said.

Two more flights scheduled to leave Tokyo's Narita airport were still undergoing safety checks, he said.