South Korea's military rulers are preparing to jettison the last fragments of civilian rule by establishing a "revoluntionary council" to run the country after the showdown with rebellious Kwangju, according to reports reaching Washington last night.

The reports, which could not be officially confirmed but were taken as credible by U.S. officials, are the latest in a stream of developments that had led to deepening pessimism here about the stability and security of one of this country's most important allies in Asia.

Faced with what is admittedly a fluid and dangerous situation, Washington officials were cast in the role of looking on helplessly while another strategically placed ally is afflicted with political instability and the threat of destruction from within.

The officials continued to maintain, as they did last week, that Washington retains a large measure of clout to affect the outcome in Korea. But the United States has not been able to use that clout effectively since the assassination of President Park Chung Hee last Oct. 26 set in motion a power struggle for succession.

The latest example of American reluctance to become involved was the State Department's negative response yesterday to appeals for mediation from the rebel forces in Kwangju. An Associated Press report from the surrounded city quoted a rebel spokesman as appealing to the United States to "exercise influence on the [Korean] government" and as suggesting mediation by U.S. Ambassador William Gleysteen.

"We have seen the press reports [but] we have no other information," said State Department spokesman Susan Pittman. "In any case, it is difficult to see how a foreign government can intervene helpfully in a situation such as that in Kwangju."

State Department officials reported that four American Peace Corps volunteers and about 20 American missionaries are believed to remain in the city. With the withdrawal of the last official of the International Communications Agency (Formerly U.S. Information Agency), no Americans with political responsibilities remain.

It is unclear whether the showdown in Kwangju will lead to an immediate spread of dissidence and rebellion elsewhere in Korea or whether the shock effect will bring about at least a few days' pause in the international ferment.

What seem increasingly clear, however, is the determination of the new Korean military leadership under Lt. Gen. Chon Doo Hwan to asset itself against any internal challenge. This military group, composed largely of generals from the late president Park's home province who rose to favor under his tutelage and shaped themselves in his image, decided on the afternoon of May 17 to take power under total martial law and impose it on the existing civilian government.

The crucial decision was taken without consultation with the United States, which considered it an overreaction to events at that time and subsequently protested the decision to the Korean leaders.

The martial law takeover and the arrest of many dissidents and political leaders, including veteran opposition leader Kim Dae Jung, touched off the rebellion in Kwangju, the capitol of Kim's home province and historic center of dissidence from Seoul's rule.

The reports that the Seoul generals are preparing to form a "revolutionary council" suggested that they have been unable to line up enough reliable and pliable civilians to maintain the facade of a civilian government working within temporary martial law.

Among the indications that Chon and his fellow generals are determined to impose their will at all costs are the arrest of Kim Jong Pil, who served as prime minister under the late president and is his successor as chairman of the ruling Democratic Republican Party, the refusal to allow meetings of the civilian parliament and continuing tight control of the Korean news media.

However, U.S. officials are skeptical that South Koreans will accept a return to the authoritarian ways of the Park regime at a time when there is widespread public sentiment for political liberalization. The attempt to impose such rule against the opposition of the center as well as the left in the Korean body politic, and in the face of intensified regional opposition, could lead to growing internal turmoil.

Among the potential consequences of an unremitting drive to seize and hold political power against strong opposition are fissures between the military and the Korean masses, eventually generating dissidence and possibly even mutiny in military ranks. Such a development could lead to Vietnam-like rebellions and insurgency within South Korea, which continues to face a serious external threat from communist North Korea.

"The situation is dangerous almost beyond words," said an informed U.S. official. He added that it is the most dangerous situation to confront a U.S. ally in Asia since the fall of Saigon in 1975.

Faced with the assertion of power by the Korean generals 10 days ago and the eruption of rebellion in Kwangju, the United States publicly urged "maximum restraint" by all parties to resolve the civil strife, even while making clear its opposition to the departure from the previously established path of political liberalization.

Beyond the words, the main U.S. deeds were the dispatch of two airborne command post planes and a naval task force headed by the aircraft carrier Coral Sea to the area near the Korean peninsula, as a warning to North Korea not to intervene.

The U.S. commander in Seoul, Gen. John Wickham, also approved a request by the Korean military to remove four South Korean regiments from the general reserve of the joint U.S.-Korea forces for use in riot control. This action seemed to place the United States in sympathy with the military forces seeking to quash the insurgency, but Washington officials said Wickham had no choice. He could have disapproved the request only if it would have jeopardized South Korea's external security, officials said.

With 39,000 U.S. troops and enormous economic, political and strategic stakes at risk in Korea, Washington opted last week to place security and public order at the top of its immediate priority list. This meant, in practice, holding back any serious effort to challenge the Korean generals on their broader course until the rebellion in Kwangju was over.

How the United States will proceed after Kwangju is retaken is still undecided. Senior State Department officials said Washington has not abandoned its goal of a return to a popular and liberalized political regime in Korea. However, it is far from clear what the United States can do, amid deepening internal conflict within Korea, to accomplish this end.