JAPAN WARNS U.S. IT MAY RETALIATE IN TRADE DISPUTE Japan warned the United States it may take retaliatory measures if the United States imposes its planned trade sanctions on April 17, a senior government official said. Shinji Fukukawa, Vice Minister of the International Trade and Industry Ministry, said in a statement Japan would consider measures under the General Agreement on Tariffs and Trade and other actions if the United States imposes 100 pct tariffs on some Japanese exports as planned next week. However, Fukukawa said Japan was ready to continue trade talks with the United States despite its failure to convince America to call off the threatened tariffs during two days of emergency talks which ended in Washington yesterday. Last month President Reagan announced the sanctions in retaliation for what he called Japan's failure to honour a July 1986 agreement to stop dumping computer microchips in markets outside the United States and to open its home market to American goods. Fukukawa said the United States had regrettably not listened to Japan's explanation of its efforts to live up the pact and said Washington had not given any detailed explanation of why it planned to impose the tariffs.