The United States publicly called on South Korea yesterday to resume progress "toward the establishment of a broadly based civilian government" amid strong indications that the military leadership in Seoul is preparing to do exactly the opposite.

The U.S. statement by State Department spokesman Thomas Reston put Washington on the line in opposition to reported plans by Korea's generals to create a militarized government under a powerful leadership council.

The major question under discussion in official circles here was what the United States should do if -- as seems likely -- its advice is rejected

The immediate U.S. response to a new power play by Lt. Gen. Chon Doo Hwan and his fellow generals is likely to be a new statement here that it is under study, according to U.S. sources.

In fact, to top-level Carter administration meeting -- or series of meetings -- probably will be convened at that point to determine how far Washington will go, and at what risk, to challenge the Korean generals.

Washington's concern goes well beyond philosophical opposition to a repressive military takeover of a major allied nation where American combat forces are stationed. The most serious concern among policymakers arises from doubt that Chon and his comrades will be able to impose military rule on the rest of South Korean society, in present circumstances, without explosive consequences risking widespread instability.

Officials expressed relief that Korean military operations in the rebellious city of Kwangju did not end in the bloodbath that had been feared. At the same time, it seemed clear that the end of the Kwangju revolt was not the end of South Korea's problems, given the apparent determination of the Seoul generals to continue to accumulate and consolidate their power.

The State Department declaration by Reston expressed "regret that the situation [in Kwangju] reached the point it did."

Reston went on to say, "Now that relative calm is returning, we believe it is most important that underlying issues be addressed in a spirit of reconciliation by all elements of Korean society, and that progress be resumed toward the establishment of a broadly based civilian government."

The State Department position was in keeping with decisions made last Thursday afternoon in a meeting of the National Security Council's "policy review committee," chaired by Secretary of State Edmund S. Muskie. "Among others participating were Secretary of Defense Harold Brown, presidential assistant Zbigniew Brezinski and CIA Director Stansfield Turner as well as their top assistants for Asian affairs.

The upshot of the meeting, according to informed sources, was a two-phase U.S. policy.

Phase one, during the occupation of Kwangju, was to counsel moderation while emphasizing the need for security and public order.

Phase two, following the end of the Kwangju drama, was the call for resumption of the drive for a more open and representative government.Such a call flies in the face of a bid by Chon and other senior generals to exercise power to the exclusion of the National Assembly, political parties and other civilian leaders.

There was no indication that last Thursday's meeting seriously addressed the question of how far the United States will go, beyond public and private advice, to combat a military takeover in Seoul. When and if a new moment of decision comes, U.S. policymakers will have to deal with a long and complex list of political, economic, strategic and historic stakes and interests in South Korea.

Pentagon sources said it has not yet been determined how long the naval task force led by the aircraft carrier Coral Sea will remain on station in the area as a warning to North Korea against attempts to exploit the instability in the south. The Coral Sea had been headed home from duty in the Indian Ocean when it was diverted as a show of force as South Korea's internal crisis erupted.

Two airborne command post planes also dispatched to the area last week are expected to remain for the indefinite future. The United States had planned to station the two aircraft on Okinawa on a long-term basis beginning early next month, but advanced the deployment as a symbol of Korea-related resolve, the sources said.