Leaders of warring groups of Nicaraguan Miskito Indians said yesterday they have formally united to resist exploitation by both the Sandinista government of Nicaragua and the U.S.-backed anti-Sandinista rebels.

The union, concluded Monday after a three-day meeting and announced yesterday, involves these groups:

*Misura, an alliance of Miskito, Sumo and Rama Indians who have been fighting alongside the rebels.

*Misurasata, which also has been fighting the Sandinistas but recently has tried to reach an accord with them.

*A relatively unknown group called the Southern Indigenous Creole Communities (SICC).

*A few independent village representatives.

If the union is confirmed in a general Miskito assembly scheduled next month in Honduras, and if it manages to adopt a bargaining position distinct from the Sandinistas and the rebels, it could constitute a new power factor in the Nicaraguan conflict.

The Miskitos, living in scattered impoverished villages along Nicaragua's south and east coasts, have been ignored by the mainstream Nicaraguan culture and economy for generations. They backed the Sandinista resistance to dictator Anastasio Somoza in hopes of pressing claims to regional autonomy, but the Sandinistas, after taking power, forcibly relocated the Miskitos and subjected them to what the Miskitos called an effort to eradicate native traditions and languages.

Many of the Indians took up arms as counterrevolutionaries, or contras, without formally joining the Nicaraguan Democratic Force (FDN), while thousands of others fled to Honduras. Leadership battles split the Miskitos into several groups, each claiming to control 3,000 to 4,000 armed men.

After Congress cut off U.S. aid to the contras in May 1984, Indian leaders began to complain that they were not receiving any of the private funding that flowed in to keep the FDN going.

Misurasata leader Brooklyn Rivera opened peace talks with the Sandinistas six months ago, but negotiations broke off last month with each side saying the other was unreasonable.

Meanwhile, Misura official Wycliffe Diego was in Washington to say that the FDN and other Nicaraguan resistance figures have provided inadequate funding and have refused to include Misura leaders in policy decisions. "They are all taking advantage of our divisions," he said in an interview.

For example, Diego said, no Miskito representative was involved in the June 12 announcement by resistance leaders Arturo Cruz, Adolfo Calero and Alfonso Robelo of a new organization called the Unified Nicaraguan Opposition. The three, whom Diego called "the triple-A," said the UNO would "channel the efforts of democratic Nicaragua on all the battlefronts" and provide "just solutions for the demands of ethnic minorities."

Armstrong Wiggins of Misurasata, who handles Indian human rights issues for the Washington-based Indian Law Resource Center and who attended the June meeting in Miami, said the term "ethnic minorities" was an insulting indication that the contra leaders "use the same language the Sandinistas use" in referring to the Indians.

The Miami communique named Rivera, Diego and Jenny Lee Hodgson of SICC to organize the July Honduras gathering.