A U.S. call for the Organization of American States to step into the Nicaraguan civil war and investigate allegations of atrocities there ran into stiff opposition yesterday from Latin America's conservative military regimes.

At a special foreign minister-level OAS meeting to discuss Nicaragua, delegates from several military-dominated governments attacked the U.S. proposal as a violation of the OAS charter's prohibition against intervening in the affairs of any hemispheric country.

The U.S. proposal, presented to the OAS Thursday night as a resolution, called for sending the OAS human rights commission to Nicaragua to probe the atrocity charges and for OAS member countries to be prepared to act as mediators in the conflict between Nicaraguan President Anastasio Somoza and his domestic opponents.

However, most of the military governments in the 25-member OAS appeared to be rallying instead around a resolution offered by El Salvador. It called for humanitarian aid to the victims of the Nicaraguan bloodshed but ruled out any political action by the OAS in that Central American country.

In an effort to resolve the impasse, the meeting appointed a working group to seek a compromise acceptable to both sides. The negotiators, who were working late into the night, were instructed to report to a formal session of the meeting scheduled for this morning.

However, as of last night, diplomatic sources were pessimistic that a compromise resolution capable of gaining the 17 votes needed for passage could be achieved.

The sources pointed out that the military governments apparently will not budge from their opposition to any steps that might be construed as intervention. Similarly, the sources added, the United States and those democratic Latin American countries hostile to Somoza are unlikely to accept a resolution without any teeth.

The United States repeatedly has advocated mediation as the best way to halt the bloodshed in Nicaragua; and, at a White House meeting yesterday, President Carter and his national security affairs adviser, Zbigniew Brzezinski, told a group of visiting editors that mediation by Nicaragua's "friends and neighbors" was a goal that Washington intends to pursue.

In the discussion, Brzezinski defined mediation as "some process of political accommodation to create resolution of the differences that led to the dispute."

Other U.S. officials privately expressed concern that if the attempt to start a mediation process fails, the tensions created in Central America by the Nicaraguan conflict could increase.

In fact, the officials said, there was concern within the administration late Thursday and early yesterday about intelligence reports that Venezuela and Panama, two countries hostile to Somoza, might be preparing to intervene militarily in the Nicaraguan conflict.

By late yesterday, however, the officials added, these fears largely had been quieted by newer and better intelligence. The more likely expectation, the sources said, was the Venezuela, if it considers the OAS unable to deal with the situation, will attempt to raise the Nicaragua question in the United Nations.

Defense Department sources revealed that the guided-missile cruiser Richmond K. Turner had been sent to stand off Nicaragua's Pacific coast to monitor developments in the fighting by intercepting radio messages.

In Managua yesterday, the Nicaraguan National Guard issued a statement accusing "communist guerrillas" of committing atrocities against the civilian population, adding that the guerrillas "wore National Guard uniforms so that these crimes could be attributed to our soldiers."

In another development yesterday, the Senate voted to delete from the fiscal year 1979 foreign aid appropriations bill a $150,000 grant for training of Somoza's military force, the National Guard. If the deletion stands in the final version of the bill adopted by Congress, Nicaragua no longer would be getting any U.S. military assistance.