MANAGUA, NICARAGUA, DEC. 21 -- Rebels claimed major gains today in a drive into Nicaragua's remote mining region three days before the start of a Christmas truce, but the Defense Ministry said most of the casualties were civilians.

The ministry also said a surface-to-air missile fired by the U.S.-supplied forces, known as contras, hit a cargo plane of the state-owned Aeronica airline, wounding four crew members.

The plane, en route from Managua to Panama, made an emergency landing at a farm in northeastern Costa Rica and an investigation was begun, Costa Rica's Public Security Ministry said. There were unconfirmed reports that mechanical trouble, rather than a missile, forced the plane down.

A contra spokesman in Miami, Jorge Rosales, said the offensive "demonstrates . . . we are a force in Nicaragua that has to be taken into consideration." President Daniel Ortega told the National Assembly the contras staged it "with clear propaganda purposes."

Despite the offensive, high-level representatives from the contras and Nicaragua's leftist Sandinista government gathered for a second time in Santo Domingo, capital of the Dominican Republic, to discuss a cease-fire in the 6-year-old civil war.

Both sides already had accepted a Christmas truce proposed for Thursday and Friday by their mediator, Nicaraguan Cardinal Miguel Obando y Bravo. The cardinal said Monday night he did not expect the fighting to affect the truce.

The fighting began before dawn yesterday in the area where gold and silver are mined in the north, near the Caribbean coast.

A brief government report said 53 civilians were wounded in the village of Siuna, where it said rebels blew up a gasoline storage tank. The report said fighting continued in Bonanza village, but all was "under complete control" in Siuna and Rosita.

The villages were the targets of what the contras said was their largest offensive of the war.

President Ortega said during a morning appearance at the National Assembly, "They continue fighting in the zone."

The Defense Ministry said four rebels died while Sandinista troops suffered 10 killed and wounded at Siuna. It said 20 rebels and 13 soldiers died at Rosita.

An agreement was reached yesterday between Congress and the White House that would give the contras $8.1 million in nonmilitary aid through the end of February.

The Reagan administration described the attack by 1,000 rebels as "very encouraging" and "very helpful." Presidential spokesman Marlin Fitzwater said the contras "remain a strong fighting force."

"Military pressure has been largely responsible for forcing the Sandinistas to come to the bargaining table," said State Department spokeswoman Phyllis Oakley. "We believe military pressure, coupled with negotiations, will compel the Sandinistas to live up to their commitments under the {regional peace} accord."

Ortega said, "If the American Congress approves new mercenary funds . . . it will be making a mockery of the sovereignty of the Central American presidents." The region's five presidents in August signed a peace plan that won the Nobel Peace Prize for Costa Rica's president, Oscar Arias.

Contra spokesmen said 7,000 rebels mounted yesterday's offensive. They claimed at least 100 Sandinista casualties, without specifying the number killed. They also said nothing about rebel casualties. The contras said they overran one village and nearly captured a second.

The Defense Ministry also reported that two women, five children and four civil defense members died in a separate, pre-dawn raid yesterday on a cooperative near Rama in central Nicaragua, about 185 miles southeast of Managua.

The mining region has become important for the government as it tries to bolster a declining economy. The isolated region also is believed to contain air defense installations, artillery and tanks to protect the mines.

{Bosco Matamoros, a spokesman for the Nicaraguan Resistance, said by telephone from Santo Domingo that one of the targets of the offensive was a listening post manned by East Germans to monitor broadcasts from as far away as the U.S. southern coast.}

The contras' biggest operation reported previously came in mid-October when 800 rebels unsuccessfully tried to cut off a 40-mile stretch of the Rama Road, Nicaragua's main strategic highway.

The villages attacked yesterday are in the jungle-covered province of Zelaya. There are no telephones to the battle area. Communication is by military radio, which operates sporadic.

The Defense Ministry said no travelers would be allowed into the area by land.

The push was first announced in Santo Domingo by Matamoros. "We began the largest military operation of the war with approximately 7,000 of our men, which involves 11 regional commands and 45 task forces," he said yesterday.