A senior Costa Rican official said today that his country's efforts to bring the guerrillas and government of El Salvador to the negotiating table have met with positive responses from both sides of the two-year-old civil war.

"We have found that they are open for discussions," said Deputy Foreign Minister Ekhart Peters in a telephone interview. "The atmosphere seems perfect for a dialogue from both sides. We think that both are willing and able to talk."

"Conversations with both groups definitely exist," said Peters in the Costa Rican capital, San Jose. He said that the effort is being pushed personally by Costa Rican president, Luis Alberto Monge, and added that his government hopes to bring top guerrillas and Salvadoran officials face to face within 90 days.

U.S. officials in El Salvador, while noting some positive movement, cautioned that there is no dialogue as such. They said the U.S. position and that of the Salvadoran government it backs have not changed. A U.S. envoy suggested that reports of a crucial softening of the insurgents' stated negotiating aims could "just be tactics on the part of the left."

The guerrilla initiative he referred to appears to have come last month when Guillermo Ungo, leader of the guerrillas' political allies in the Revolutionary Democratic Front, met with Monge. This was followed by a meeting between Costa Rican Foreign Minister Fernando Volio and Salvadoran Interim President Alvaro Magana two weeks ago.

United Press International quoted Magana as saying, "I have not received any messages from anyone" among the guerrillas; "There has been no change in the situation" on negotiations.

The moves come as several moderate Latin American leaders are expressing increased worries that the civil conflicts in Central America could lead to wars between nations. Fighting in the region includes, besides the Salvadoran civil war, a bloody civil conflict in Guatemala and border confrontations between Honduras and Nicaragua.

The presidents of Mexico and Venezuela wrote letters to President Reagan and the leaders of Nicaragua and Honduras earlier this week trying to help those two countries begin negotiations. One well-informed Central American official said the letters, which were not released, may include a positive response to Washington's endorsement of proposals to demilitarize the region.

Regarding the specific problems of Salvadoran talks, a U.S. official noted what his embassy regards as the apparent omission in public statements by a member of the guerrillas' political front of past demands for a restructuring of the government's armed forces so as to include elements of the insurgent armies.

The guerrillas reportedly have limited their goals to an assurance of safety for leftists, an end to the government's state of siege, access to the press and a reopening of the national university closed two years ago. If so, the U.S. official said, there might indeed be room for serious talks.

Salvadoran guerrilla spokesmen in Mexico City declined direct comment and noted that while they have proposed negotiations without previous conditions for more than a year, Salvadoran Defense Minister Guillermo Garcia appeared to reject unequivocally talks as recently as this week. "Neither the government nor the armed forces of El Salvador will negotiate with the guerrillas or their allies," Garcia reportedly told reporters.

Salvadoran leaders make a distinction between negotiations leading directly to guerrilla participation in the government, which is rejected, and dialogue leading to the participation of the guerrillas or their political organizations in the electoral process. A vote for the current Constituent Assembly was held March 28 despite guerrilla efforts to stop the polling. Presidential elections are tentatively scheduled for the spring of 1984.

The United States has supported and continues to support the Salvadoran government in this fundamental distinction, said an informed U.S. official reached in San Salvador.

The State Department issued a statement Friday affirming that U.S. policy on talks remains unchanged. Noting that leaders of the parties in the Salvadoran Constituent Assembly have joined in "a united action plan to end divisions in that country," it added: "We are encouraged" that through a Peace Commission established under the plan, the Salvadoran government "is in fact exploring ways to bring about reconciliation. . . ."

Peters said that elections "naturally would be the end result of any conversations" contemplated between the combatants in El Salvador.

Spokesmen representing all elements of the guerrillas have said that elections would be held after negotiations but they regarded a change in the composition of the government and the Army as necessary before a fair vote could take place.

Peters declined to name the "representatives at a high level" talked to on both sides of the conflict. He said four of the five guerrilla factions have participated.