A cryptic remark by South Korean President Chun Doo Hwan and two weeks of inaction by that country's supreme court have heightened speculation in official circles here that the life of the most prominent Korean opposition political leader will be spared, thus averting serious damage to Washington-Seoul Relations.

The opposition political leader, Kim Dae Jung, was sentenced to death on sedition charges by a military court in September. That verdict, now before the Korean Supreme Court for review, touched off alarm in the United States and Japan, where Kim is well known as a democratic figure, and in many other nations.

Among many public and private expressions of concern, one of the weightiest was a face-to-face plea in Seoul last Friday by Defense Secretary Harold Brown, who had traveled thousands of miles for the main purpose of discussing the case with the Korean president. In that private meeting, according to U.S. sources, Chun remarked to Brown that "our courts are fair -- let the verdict stand."

Some American officials took his as a hint that the supreme court is likely to throw out the death sentence in its review of the Kim verdict. This belief had been encouraged by the fact that the court has not yet ruled, despite very widespread rumors about two weeks ago that a ruling upholding the death sentence was about to be announced.

Another interpretation, however, is that Chun was telling the United States by that remark that he will not exercise his power of executive clemency if and when the supreme court upholds a death sentence. It is assumed in official circles here that the high court, like other institutions in Korea, is susceptible to influence if not to outright orders from the Blue House, the local equivalent of the White House.

The Carter administration made it clear to Seoul even before the original death sentence that killing Kim would have the most serious consequences for U.S.-Korean relations, in part because U.S. public antipathy to such a course could sour the alliance for a long time. A few days after the November election, President-elect Ronald Reagan authorized a private message to Chun endorsing the same position.

Many members of Congress of both political parties have communicated their concern privately to Chun since the death sentence was announced in September. In keeping with State Department advice, most of these communications, from the leadership of the foreign relations committees and from many individual members, have been kept quiet to avoid charges of outside interference in Seoul.