TOKYO -- Three young women who disappeared mysteriously from beaches 10 years ago may have been kidnaped to teach Japanese language and customs to North Korean agents, police here suspect.

Police reopened the cases this week based on the testimony of Kim Hyon Hui, 26, who confessed in Seoul to being a North Korean saboteur who placed a bomb on a South Korean jetliner that is believed to have exploded in midair, killing all 115 aboard, last Nov. 29.

Kim told investigators that she had been trained for two years to pass as a Japanese by a 31-year-old Japanese woman who had been kidnaped while playing on a Japanese beach. Kim identified her only by the Korean name Un Hye.

Investigators from the National Police Agency will travel to South Korea this week with photographs of the three missing women, who were between 22 and 24 years old when they disappeared during the summer of 1978, to see if Kim Hyon Hui can identify her tutor. Japanese police also will seek to learn how Kim obtained a false Japanese passport in the name of Mayumi Hachiya.

The case poses delicate diplomatic problems for Japan, which does not maintain relations with communist North Korea, but which tries not to offend either Korea because of its role as colonizer of the peninsula from 1910 until 1945. Japan and North Korea are in the midst of a separate diplomatic standoff involving North Korea's seizure of two Japanese merchant seamen, in apparent retribution for Japan's refusal to return a defecting North Korean.

In the case of the sabotaged South Korean plane, Kim confessed in Seoul that she and a male companion -- another suspected North Korean agent -- traveled on the first leg of the Korean Air Flight 858, from Baghdad to Abu Dhabi, and got off before the jet took off for Bangkok and Seoul. When police attempted to question them after the jet had disappeared somewhere near Thailand, they both swallowed cyanide capsules. The older man died almost instantly and the woman fell ill.

The young woman was extradited to South Korea and interrogated for a month until, according to South Korean investigators and her own testimony at a news conference last Friday, she realized she had been led astray by her North Korean trainers. She identified herself as Kim Hyon Hui, daughter of a North Korean diplomat, and said she had been trained for more than seven years for her secret mission.

Part of the training, she said, included living with a Japanese woman in a guest house outside Pyongyang from April 1981 to March 1983. There, she said, the woman known as Un Hye trained her in the finer points of Japanese language and manners.

When she was apprehended by police, Kim alternately claimed to be Chinese or Japanese and said she could not speak or understand Korean. But she made several slips, including claiming not to recognize a kind of seaweed known here as "nori," a common food in Korea and Japan but which Kim apparently believed was eaten only in Korea.

Nonetheless, Kim's story prompted police here to search their files for missing women who might fit her description. They focused on three women who disappeared from beaches on the Japan Sea, facing Korea, on July 7, July 31 and Aug. 12, 1978.

All three disappeared with their boyfriends and in each case their cars or bicycles were left untouched, leaving police with no clue as to motive.

On Aug. 15, in a possibly related case, four men attacked yet another woman and her fiance as they walked along a Japan Sea beach, but the pair managed to escape.

Japanese patrol vessels have spotted, but never managed to approach, white speedboats operating in the Japan Sea between Japan and North Korea, police said.