J. William Middendorf II, U.S. ambassador to the Organization of American States (OAS), said yesterday he will ask it to deal directly with the Sandinista government of Nicaragua if the Contadora process fails to bring peace to the region.

"My government does not intend to allow this organization to ignore its responsibilities in this regard," Middendorf said, "and reserves the right . . . to introduce a resolution leading to the satisfactory resolution of the Nicaraguan problem."

Under the Rio Treaty of the OAS, hemispheric nations can impose economic and diplomatic sanctions, or the use of armed forces, against a country found guilty of aggression.

In his 30-minute speech to an OAS council meeting, Middendorf accused the Sandinistas of human and civil rights violations, religious persecution, threatening the livelihood of civilians, and allowing malnutrition among the children of Nicaragua. He described elections the Sandinistas held last year as "a sham."

"Can we assume that promises not to attack their neighbors will be kept by the Sandinistas?" Middendorf asked.

In a sharply worded response to Middendorf's remarks, Edgard Parrales, Nicaragua's OAS ambassador, said, "It is not we Nicaraguans; international law itself is opposing the covert actions the U.S. is carrying out there." Parrales read at length from reports of alleged rapes and other violations by U.S.-backed Nicaraguan "contra" rebels. He said the U.S. ambassador's accusations were based largely on "irresponsible" and "superficial" reporting, and he accused the United States of deliberately undermining the Contadora process.

Middendorf's speech came as Congress prepares to vote on the Reagan administration request for $14 million in additional aid for the contras. It also follows a congressional resolution supporting the contras, and a speech by Sen. David F. Durenberger (R-Minn.) calling for the OAS to intervene in Nicaragua.

Reminding OAS members that Nicaragua has failed to live up to promises it made when the OAS decided to support the Sandinista-led government nearly six years ago, Middendorf said, "They promised to hold early free elections, to establish an independent judiciary, and to uphold human rights."

Middendorf then detailed the detrimental impact he believes Sandinista rule has had on Nicaraguan civilians. Children as young as 11 are taken off streets to join the military, religious leaders are debased as "counterrevolutionary," Miskito and other Indians are mistreated and censorship is rampant, he said.

The Contadora group -- Panama, Venezuela, Mexico and Colombia -- is due to meet next month to seek a solution to the Central American problem, Middendorf said, and "foolproof measures of verification must be included in any such agreement before it is effective."

In his rebuttal, Parrales emphasized his government's support of the Contadora group.