Angolan rebels said today they attacked key installations at a joint U.S.-Angolan oil complex, inflicting heavy damage on the main support of the country's war-shattered economy. But a U.S. oil company official in Angola denied the claim.

The alleged attack on the American-operated Gulf Oil complex in the northern enclave of Cabinda came two months after the Reagan administration approved $10 million to $15 million in covert aid for the rebels, the National Union for the Total Independence of Angola (UNITA).

UNITA has been fighting a 10-year campaign to topple the Soviet and Cuban-backed Angolan government.

A rebel spokesman in Lisbon said guerrillas shelled oil storage tanks at the central depot of the Cabinda Gulf Oil Co. in a predawn raid yesterday and dynamited four locations on a pipeline linking the depot to offshore wells. He added that areas of the installations were still burning today.

A Gulf official contacted by telephone at the depot at Malongo in Cabinda, where about 150 Americans work, said shooting had been heard outside the complex last night, but added that he knew of no damage to the main oil installations. He said there were no casualties.

The official, who spoke on condition of anonymity, said the pipeline allegedly attacked by the rebels was not in use so that the company would not necessarily know if it had been sabotaged.

The UNITA spokesman insisted that the rebels had inflicted "extensive damage" on the central oil installations. He said it was not in the interest of Chevron Corp., which owns Gulf Oil, to disclose the extent of the sabotage.

Charles Wooton, a spokesman at Chevron headquarters in San Francisco, said neither the oil complex nor the pipeline had been hit. "Our installations are intact," he said.

A spokesman for the State Department, Robert L. Bruce, said U.S. officials had not confirmed that the attacks occurred. "We are concerned about threats to American lives, whatever the source," he said, "and we would of course be unhappy if the reports were true."

When UNITA leader Jonas Savimbi visited Washington in January he reportedly warned the adminstration that, despite U.S. aid for UNITA, he would be forced to attack Cabinda's oil installations because they were the sole remaining support of the Angolan economy. He said he would try to spare American lives in any such attack, according to diplomats here.

The rebels raided another key economic target on March 1 when they seized more than 190 foreign hostages and a month's production of diamonds worth an estimated $6 million. The foreigners were released unharmed in Zaire last Saturday after a 200-mile forced march.