In a surprise reversal, the Nicaraguan government today admitted that U.S.-backed guerrillas had sabotaged underwater facilities at a vital oil terminal six days ago.

Daniel Ortega, coordinator of the ruling revolutionary junta, said in a speech that saboteurs "trained and sent by the CIA" had used explosives to damage offshore equipment where tankers unload crude oil at the Pacific port of Puerto Sandino, 35 miles northwest of here.

Ortega said that the facilities were being repaired, and it was not clear how much the attack would reduce Nicaragua's oil supplies. A tour of the onshore facilities Saturday showed no sign of damage, and oil that had been unloaded from a tanker about a week earlier apparently was flowing through a pipeline to the nation's only refinery outside Managua.

By preventing tankers from unloading additional crude oil at the terminal, however, the attack may force Nicaragua to tighten rationing or to import refined products.

In addition, the attack's success demonstrates that the Nicaraguan Democratic Force has the ability to strike economic or other targets far from Honduras where the rebels are based. The force announced the attack last Thursday, the day it happened, and its radio station later reported that frogmen had carried it out.

Leaders of the force, which receives aid from the Central Intelligence Agency, have said they want to carry their fight from the countryside to the nation's cities and towns.

Ortega's announcement came five days after the Defense Ministry's chief spokesman said that there had been no attack at Puerto Sandino. An aide at the Defense Ministry's press office said that no spokesman would be available today to comment on why the government waited so long to acknowledge the attack.

Nicaragua currently is receiving virtually all of its crude oil from Mexico, and Mexican diplomatic sources have said the flow is an average of 7,500 barrels a day. Tankers arrive at Puerto Sandino about twice a month, dock in the bay and pump their oil through an underwater pipeline to storage tanks on shore.

Ortega said that a "brigade" of commandos had sabotaged facilities of the terminal that were 300 yards from the coast and 60 feet underwater. He said they had destroyed some buoys and damaged the underwater pipeline.

"This can give us an idea of the technical means used by the CIA," he said.

The Nicaraguan Democratic Force said in announcing the attack that it had been carried out by an officer in the Sandinista Army who was secretly working with the guerrillas and by "an adviser." Warning that there would be more attacks, the force said that workers trying to repair the facility or tankers unloading more oil supplies would be considered "targets of war."

The Puerto Sandino terminal and the Managua refinery are considered prime targets for economic sabotage. Motorists already must obtain rationing coupons to buy gasoline here, with the average motorist limited to 20 gallons for all of last month.

Adolfo Calero Portocarrero, a leader of the Nicaraguan Democratic Force, said in Washington that rebel forces had made a similar attack on a pier at a spot called Padre Ramos on the Pacific coast across the Gulf of Fonseca from the Salvadoran port of La Union. U.S. intelligence sources have identified Padre Ramos as a place from which Nicaragua allegedly ships arms to leftist rebels in El Salvador.

Mexico has been supplying Nicaragua even though this country has fallen behind in its payments. The Mexicans have supported the Sandinista revolution and have criticized recent rebel attacks here.

Mexico's position has differed from that of Venezuela, which cut off oil supplies to Nicaragua a year ago when Nicaragua fell too far behind in its payments.

A senior Western diplomat here said that the Nicaraguans have been stockpiling oil in anticipation of guerrilla attacks, and another source estimated that the country had several months' supplies in tanks.