Washington — The United States will remain ''absolutely neutral,'' US officials say, if South Korea's opposition leader Kim Dae Jung returns to Korea as he vows to do by the end of the year.
But the cool, brief, and cautious comments coming from officials on this subject mask a certain anguish. Although they will not say so openly, officials here are clearly concerned that Mr. Kim's return will trigger turmoil in Korea, a nation considered strategically important to the US.
It is ''quite plausible,'' one State Department official said, that Kim's return to South Korea would bring about his arrest and imprisonment.
The Korean government has yet to make any formal statement on the subject. But a Korean Embassy official asserted that Kim Dae Jung is still on a list of ''banned politicians'' and therefore could not be allowed to resume political activities in Korea.
''He is still bound by a suspended sentence,'' the Korean official said, referring to a 20-year prison sentence which Kim was given in 1980 following a crackdown on political dissent. He was in jail for 2 1/2 years.
The official added that now, ''no one can say what will happen to his legal status. It's a legal question.''
Despite what the embassy official said, however, the future status of Kim Dae Jung is also an emotional and political issue. The emotion comes in part from the example of Benigno Aquino, the slain Filipino opposition leader. There are uncanny similarities in the backgrounds of Aquino and Kim Dae Jung. Both spent time in prison, went into exile, and ended up as research fellows at Harvard University. Aquino was assassinated last year immediately upon his return to Manila.
On Wednesday, the exiled Kim Dae Jung announced that he had informed the US and South Korean governments that he had decided to return to South Korea by the end of this year. He gave several reasons for returning, including completion of his Harvard fellowship tenure and the medical treatment which he had sought here. Kim said in a statement that he was deeply worried about ''trends toward desperation and radicalization'' on the part of some Koreans. He said that he wanted to open a dialogue with President Chun Doo Hwan's regime.
In an interview here, Kim said that while there were similarities between his case and that of Benigno Aquino, he did not think that the Chun government would be ''so stupid as to commit the crime'' which was committed against Aquino at the Manila airport.
In late 1982, the Chun government released Kim from prison and allowed him to go into exile. The United States government clearly played a role in securing his release, just as it had in encouraging the Korean government to commute the opposition leader's death sentence. But despite the assistance from the US, Kim argues that the Americans have not done enough to support the development of democratic processes in South Korea.
Elliott Abrams, assistant secretary of state for human rights and humanitarian affairs, argues, however, that there has been a ''pattern of improvement'' in the human rights situation in South Korea over the past 31/2 years.
Kim Dae Jung met with Mr. Abrams recently, and, by Kim's account, the assistant secretary showed great concern for many of the problems Kim touched on. According to the Korean Institute for Human Rights, Abrams stated that while the US government could not intervene when it came to Kim's planned return to Korea, he would follow that development with profound concern.
In a telephone interview, Abrams said that the US wanted to stay ''absolutely neutral'' in the Kim case but that its interest in that case has been ''very clear'' since the Carter administration and on through the Reagan administration.
''I told Kim that there would be a possible threat to his security not from his government but from the government of North Korea,'' said Abrams.
Abrams said that the Communist North Koreans might view the assassination of Kim Dae Jung as a means of stirring turmoil in South Korea.
In an interview, Kim acknowledged that he might incur risks in returning, but said that he wanted to work from inside South Korea to prevent what he described as widespread disaffection among many Koreans and a process of radicalization among the country's youth.
He said he was grateful to the American government for providing for his safety and ''free activity'' in the US.
''I know that President Reagan did much to save my life,'' he said. ''I also think that I got some understanding from State Department officials . . . but I can't say that I have persuaded the American government to change its Korean policy.''