The leak of a secret United States Defense Department document in Washington last week has caused an immediate ripple reaction in South Korea. A United Press International report said that according to the 136-page classified document ''Fiscal 1984-1988 Defense Guidance,'' US defense strategy ''dictates counterattacks against North Korea, Vietnam, and Soviet coastal areas in order to tie up Soviet forces in defensive roles in the prosecution of a global conflict.''
Although there has been no official comment, Korean and English language newspapers ran the story under front-page headlines such as ''US to attack N.K., Soviet coasts if Gulf oil grabbed.''
US Army Chief of Staff Gen. Edward C. Meyer, who was visiting South Korea, did his best to smooth away the Pentagon's embarrassment. He stressed that the document was ''a guidance, not a war plan'' and that there was no chance that the US would attack North Korea without prior consultation with South Korea and Japan.
General Meyer said the thinking behind the strategy was that if the Soviets were to start a war somewhere, the US would not necessarily want to fight a war on Soviet terms. Gen. Robert W. Sennewald, top US military commander in South Korea, said that although there were no plans to increase US forces in Korea, air and naval strength and US potential to reinforce South Korea in the event of an invasion from the north were being upgraded.