Nicaraguan Defense Minister Humberto Ortega said today that more than 8,000 Nicaraguans had been killed or wounded in the fighting between the Army and anti-Sandinista guerrillas this year and that the contra forces, as the rebels are known, have entered "an irreversible process of disintegration."

In a year-end military review, the defense minister said that the contras suffered more than 5,600 casualties, including more than 4,500 deaths. He said that about 1,140 Sandinista forces were killed and that 281 civilians had died in the conflict. More than 1,300 civilians were wounded.

The death toll was the highest announced in any year since the insurgents began their war on Nicaragua's leftist government from bases in neighboring Honduras in 1982.

But Indalecio Rodriguez, a leader of the Nicaraguan Democratic Force, said the contras lost fewer than 500 fighters, including dead and wounded. He asserted that the Sandinistas lost more than 5,500.

"Our forces have never been better," he said. He said the Nicaraguan Democratic Force has stepped up recruitment of new, fighters to about 800 a month and that the contra forces now total more than 20,000.

During the press conference, Ortega, who is also a member of the national directorate of the ruling Sandinista Front, said the Sandinista Army has been improved on several fronts in 1985.

He said better coordination between the Air Force and ground troops and the addition of new highly trained counterinsurgency battalions and border guards have "pushed the counterrevolutionaries farther and farther back." He said, "The mercenary forces have entered an irreversible process of desintegration. We are certain of their fall."

In the face of weakened contra forces, Ortega said, the United States had embarked on an "interventionist escalation" that "could rapidly turn into direct military intervention."

He said escalation already has come about through the United States' supplying F5 aircraft to the Honduran Army and S7 missiles to the contras. U.S. strategy for 1986, he charged, includes plans to mine the waters off of Nicaragua's coast, strike against strategic points such as the Managua oil refinery and attempt to provoke clashes on the Honduran and Costa Rican borders that would help pave the way for a U.S. invasion.

In early December contra forces for the first time shot down a Nicaraguan aircraft. The Soviet-made MI8 helicopter, downed by an S7 missile, was carrying 14 Nicaraguans who were all killed in the crash, according to the Nicaraguan government.