Nicaragua's guerrilla-backed provisional junta charged the United States last night with attempting to undermine broad-based moderate support for its government.

The junta said that in a unilateral attempt to ensure that a new Nicaraguan government replacing President Anastasio Somoza serves U.S. interests, the Carter administration has actively sought to splinter growing cohesion among opposition groups.

The charges, by junta member Sergio Ramirez and the provisional government foreign minister, the Rev. Miguel D'Escoto, came following their meeting here Saturday night with U.S. Ambassador William Bowdler, appointed by the State Department to help ease an orderly transition of power in Nicaragua.

Instead of working toward an end to Nicaragua's civil war, however, the United States is extending it, the junta charged.

There is general agreement in most of Latin America and within the State Department that Nicaragua will not be peaceful until Somoza leaves. Late last month. the Organization of American States voted to demand Somoza's resignation and called for a democratic representative government.

The OAS stopped short, however, of calling for recognition of the five-member provisional government announced several weeks ago by Sandinista National Liberation front guerrillas fighting Somoza's National Guard.

Since then, as the junta announced its proposed government program, a number of Latin American government have made tentative contacts with it and have distanced themselves from Somoza

Mexico, Ecuador, Panama, Costa Rica, Peru and Grenada have broken relations with Somoza. Panama and Grenada have recognized the junta as Nicaragua's legitimate government. Others have hesitated, reportedly until the junta can establish a provisional capital on Nicaraguan territory and because of U.S. reluctance.

The United States has called for Somoza's replacement, first by an appointee ostensibly from the Nicaraguan Congress, then by an opposition council to be replaced by an interim government until elections.

That plan failed primarily because opposition groups in Nicaragua, including the moderate Broad Opposition Front formed to negotiate with Somoza last fall, and the Superior Council of Private Enterprise, refused to support it and instead supported the junta.

Ramirez and D'escoto charged that the United States had approached individual members of both organizations to try to persuade them to deny that support, ostensibly in exchange for power in a new U.S. sponsored government.

The State Department reportedly is concerned that the junta does not adequately represent all prominent Nicaraguan sectors, including the National Guard, a concern that infuriates the junta.

"It is really a shame the United States never showed as much solicitude for the entire population of Nicaragua so mercilessly slaughtered by the army it trained and equipped," D'Escoto said.

"Right now," D'Escoto maintained, "the United States stands as the only impediment in the Americas" to an early resolution of the Nicaraguan crisis. "The transition of power in Nicaragua is the business only of Nicaraguans, not of the United States," he said.

In a communique Saturday, the Sandinistas denied reports that it is their policy to execute National Guard soldiers and said they have offered to turn over at least 130 prisoners to the International Red Cross, but that Somoza has refused to cooperate.

The junta did not deny that a number of Somoza supporters have been killed in the 15-month-old war but said it was not Sandinista policy and that all prisoners at the end of the war would be turned over to an "institutionalized system of justice."

The junta, they said, is representative. They pointed to its recognition by Nicaragua political moderates and the inclusion of only one Sandinista member, Daniel Ortega Saavedra.

Several of the junta members have had strong ties to the Sandinista organization, but since the announcement of the provisional government they have repeatedly tried to distinguish themselves from it.

Saturday night's meeting was the second, between Bowdler and the junta, following a brief session Wednesday in Panama. Bowdler left Costa Rica last night again for Panama, and this morning was expected to travel to Washington. He is scheduled to return to San Jose on Wednesday.