In foreign policy, ties with Japan were strengthened by state visits by Chun to Japan and Japanese Prime Minister Yasuhiro Nakasone to Korea. U.S. President Ronald Reagan also paid a visit, and relations with the Soviet Union and China improved. The relationship with North Korea was strained when in 1983 a terrorist bomb attack in Burma killed 17 high-ranking officials attending memorial ceremonies and North Korea was alleged to be behind the attacks. However, in 1980 North Korea had submitted a "one nation, two system" reunification proposal which was met with a suggestion from the South to meet and prepare a unification constitution and government through a referendum. The humanitarian issue of reuniting separated families was dealt with first, and in September 1985, families from both sides of the border made cross visits to Seoul and Pyongyang in an historic event.