SAN JACINTO, Nicaragua (AP) _ Defense Minister Humberto Ortega said Nicaragua has defeated U.S.-backed rebels but will maintain its large army because of the threat of an invasion by American troops.

Ortega said Friday the rebels are ''strategically defeated'' but the government will not reduce it 80,000-member Armed Forces, the biggest in Central America.

''We can't have a lesser level,'' he said, citing the presence of rebel units across the border in Honduras and what he called the threat of U.S. military intervention.

A Contra director, meanwhile, said that a loss of U.S. support for the rebels ''could mean the blood of American soldiers.''

A transcript of a rebel Radio Liberation broadcast quoted Enrique Bermudez as saying the Contras ''are fighting for democracy in Central America.''

But Ortega said the rebels, known as Contras, were in an ''irreversible situation'' with no hope for victory.

The Contras began fighting the leftist government in 1981. The Sandinistas came to power in July 1979 in the civil war that ousted President Anastasio Somoza after 42 years of rule by the Somoza family.

Ortega spoke with reporters after addressing a crowd of soldiers at an Army Day ceremony in San Jacinto, 25 miles north of the capital of Managua.

The defense minister said the rebels still hoped to obtain renewed U.S. military aid, cut off at the end of February. He said the Reagan administration was attempting to provoke a conflict between Honduran and Nicaraguan troops to justify direct American military intervention.

The United States sent more than 3,000 soldiers to Honduras in March in response to a Sandinista border crossing into Honduras, but then withdrew the troops.

The Sandinistas and the Contras signed a cease-fire pact March 23. But high-level talks to work out the mechanics of a permanent truce in the 7-year- old war stalemated in June. Both sides accuse the other of violating the March agreement to suspend offensive military operations.

The defense minister said that the Sandinistas were ready to renew the talks but that they had to be held in Managua, the capital. The Contras refuse to return to Managua, saying they were barred from circulating freely.

Ortega repeated the Sandinista government's estimate that there are 6,000 Contras and he said 70 percent of them were in Honduras. Rebel officials say more than 10,000 combatants are in Honduras and about 2,000 remain in Nicaragua.