Concerned that a post-Somoza government proposed by leftist gurrillas would be too radical, U.S. diplomats are working to shape a moderate, more pro- U.S. alternative, according to diplomatic sources here.

The United States' effort has been rebuffed, however, by moderate business and political groups who insist they will continue to support the five-member "provisional Junta of National Reconstruction" named by the Sandinista guerrillas.

The U.S. State Department perceives the junta as containing three leftists and two moderates, a balance it considers unacceptable, U.S. sources here said. But after 20 months of fighting, during which the Sandinista rebels have suffered heavy casualties in their struggle to overthrow President Anastasio Somoza, most Nicaraguans appear to support the junta and are suspicious of American efforts to supplant or alter it.

The State Department originally hoped a rival junta could be formed that would include representatives of the church, business, Somoza's Nationalist Liberal Party, the National Guard, and the politically moderate Board Opposition Front, these sources said. But they acknowledged that this effort, collapsed during the week when leaders of the Front and the business community refused to cooperate with any attempt to undercut the rebel-backed junta, now based in Costa Rica.

After this failure, U.S. officials began campaigning to enlarge the existing junta by adding one or two more moderate members. This, they hoped, would tip the political balance within the junta to a line more acceptable to the United States, diplomatic sources said.

Again, strong support by Nicaraguans for the junta blocked the U.s. proposal. Nicaraguan sources said individuals approached by U.S. diplomats rejected any plan that involves an ultimatum to the junta. Moderate politicians expressed a willingness to consider joining an expanded junta, but only if invited to do so by the rebels.

The chief U.S. concerns, sources said, are that the junta might be too radical, and chaos could break out here following Somoza's departure. Fear of chaos led the United States to propose last week that the Organization of American States [OAS] send a peacekeeping force here as part of a negotiated solution to the civil war.

When that proposal was rejected by the OAS, U.S. diplomatic efforts turned to the Nicaraguan National Guard, the nation's only armed force, as a guarantor of public order after Somoza's exit.

This has placed the United States in the delicate position of advocating Somoza's "prompt replacement," while still hoping to retain the National Guard, which for more than 40 years has been virtually a private army of the Somoza family.

To help resolve this anomoly, U.S. diplomats reportedly accepted that the Guard should be "purified" by purging officers believed guilty of flagrant corruption or brutality. But knowledgeable Nicaraguanas say this compromise has attracted no sympathy from either the Sandinista rebels or the populace, which is increasingly bitter toward the National Guard for supporting Somoza.

U.S. Ambassador Lawrence A. Pezzullo, who arrived here Wednesday, has met with Somoza three times as part of what Pezzullo described as a "pretty intense" mediation effort. Somoza reportedly has agreed to step down, but only on conditions unacceptable to either the rebels or the United States. These include a guarantee that the National Guard be preserved in its present state and that the Liberal Party, which Somoza heads, remains an important political force.

In a concession to U.S. pressure, Somoza today agreed to free a prominent opponent, Edmundo Jurquin, who was been under house arrest since June 7. But another anti-Somoza figure, Socrates Flores, remained in custody despite pleas for his release by U.S. officials.

Meanwhile, a session of Nicaragua's Congress called for today was postponed indefinitely. The opposition Conservative Party, which holds 40 of the 100 congressional seats, refused to participate unless Congress agrees to discuss President Somoza's resignation.

With Sandinista forces regrouping outside Managua, Pezzullo is racing against time to find a way to get Somoza out of the country while ensuring that the government that replaces him is acceptable to the United States. If Pezzullo cannot find a solution soon, a new guerrilla offensive is likely to begin that could bring even more destruction to this Iowa-sized nation.

The most important cards Pezzullo has to play are U.S. recognition, which would ensure the new government's international acceptance, and reconstruction aid, which will be needed by any Nicaraguan government.

"The Americans are mistaken in their fears of a radical takeover here," said one businessman, reflecting widespread sentiment. "The new junta will definitely need strong American support. Instead of first ignoring it and then trying to undermine it, the Americans ought to be working with the junta and showing their real desire to help our country."

Three junta members in Costa Rica met briefly with U.S. special envoy William Bowdler before he departed for Panama and the United States today, Washington Post correspondent Karen DeYoung reported from the San Jose. Junta members described the meetings as inconclusive and said Bowdler asked to mean with them again when he returns to Sam Jose Wednesday.

Nicaraguans anticipate the junta may soon move in a Nicaraguan city controlled by the rebels and declare themselves the legitimate national government. This would enable them to appeal for international support, including military aid, in their fight against Somoza.

Members of the junta are former university rector Sergio Ramirez Mercado, 37; ex-physics professor Moises Hassan Morales, also 37; Violeta Barrios de Chamorro, 49, widow on an assassinated leader of the opposition Conservative Party; long-time Saavedra, 35, who shaped the alliance between the guerrillas and the middle calss; and cooking oil tycoon Alfonso Robelo, 39, a leader or the business resistance to Somoza.

The U.S. Embassy in Tel Aviv asked for the suspension in the recent talks with Israeli officials, presumably on the ground that additional arms would undercut efforts to replace Somoza with a moderate government.

In response to the U.S. appeal, sources said, Israel diverted two of its patrol boats that were en route to Nicaragua by freighter. The boats will not be delivered.

Israel also agreed to call off other planned shipments of ammunitation and weaponry to Somoza for the time being, sources said. They did not know how long the suspension will remain in effect.