Costa Rican President Luis Alberto Monge said today that his government will cut back diplomatic relations with Nicaragua to a bare minimum following what he called a "premeditated attack" by Sandinista soldiers on Costa Rican territory last week during which at least one Costa Rican Civil Guardsman was killed.

But Nicaraguan Defense Minister Humberto Ortega denied Monge's charges, insisting that he has proof that the Costa Rican guardsmen were ambushed by Nicaraguan rebels trying to provoke a diplomatic incident between the two countries.

Monge, whose government has yet to name a replacement for its former ambassador to Nicaragua who completed his tour several weeks ago, said no ambassador would be named in the near future, a sign of his country's displeasure with events of the past week.

"Relations will be at the minimum that is absolutely necessary," Monge said in San Jose, Costa Rica. Earlier this year, Costa Rica already forced Nicaragua to reduce its diplomatic personnel in Costa Rica.

Monge also called on the Organization of American States to send a commission to investigate the border incident and said he would present proof that Sandinista soldiers were responsible for the attack.

Costa Rican officials have said that Sandinista troops crossed about three miles into Costa Rican territory Friday and ambushed a 21-man Costa Rican patrol, killing one guardsman and injuring 11 others.

Defense Minister Ortega told a Managua press conference that his troops had intercepted a radio communication between anti-Sandinista rebels of the Revolutionary Democratic Alliance (ARDE) and Costa Rican authorities in which the Costa Ricans were told that Sandinista troops had crossed the border. He said when the Civil Guardsmen went to investigate, they were ambushed by ARDE forces and not by Sandinista troops, who he insisted had not crossed the border. ARDE leaders denied involvement.