USAir and Delta Air Lines reported large losses for the third quarter yesterday, mostly as a result of higher fuel prices that threaten continuing trouble ahead.

UAL Corp., parent of United Airlines, reported profit of $106.1 million for the quarter ($4.87 per share), but that was down from $110.9 million ($5.11) a year ago, and industry analysts said that the airlines are likely to produce even worse results in the fourth quarter of 1990 and the first quarter next year.

USAir Group Inc. reported a net loss of $120 million (a loss of $2.87 per share) on revenue of $1.6 billion and an operating loss of $130 million. Delta reported a net loss of $51.6 million ($1.27) -- the company's first loss for the September quarter since 1982. Delta's operating loss for the quarter was $73.6 million.

Airline earnings are being battered by high fuel prices. Although the airlines have increased fares approximately 15 percent since the oil price surge began in August, fare increases haven't caught up with higher costs, according to the industry. The fare increases have been phased in in several steps and were offset by discounts on tickets that many travelers were able to buy before the increases.

Delta yesterday became the most recent airline to adopt the latest in the round of fare increases -- a 5.8 percent hike that goes into effect Oct. 31.

UAL Corp. Chairman Stephen M. Wolf noted yesterday that United's fuel costs have jumped from an average of 61.3 cents per gallon during the third quarter last year to 73 cents this year.

"Moreover, we expect the average price to be significantly higher in the fourth quarter as the full effect of current increases are felt and as the demand for home heating oil rises with the onset of winter," he said. "We are very concerned about the effects of these increases on United's earnings."

"It's a tough outlook," said Mark Daugherty, an airline industry analyst with Dean Witter Reynolds Inc. in New York, who noted that several airlines have sold assets recently to raise cash. "There are some ominous signs there."

Robert J. Joedicke of Shearson Lehman Brothers Inc. in New York said that the industry is likely to lose as much as $1.5 billion this year.

Both USAir and Delta reported larger losses than some analysts had been anticipating. USAir Group Chairman Edwin I. Colodny said that a 23 percent increase in the average cost per gallon of jet fuel was the biggest factor. The airline has had a net loss of $233 million on revenue of $4.9 billion for the first nine months of 1990, compared with net income of $39 million on revenue of $4.8 billion a year ago.

The third quarter results for USAir included a one-time depreciation charge of $36 million for 24 older, fuel-inefficient aircraft that are being phased out of service, and a $14 million accrual for merger-related costs.

UAL recorded a $93 million gain on sales of 12 aircraft during the quarter, but it also incurred $30 million in expenses related to an attempted employee acquisition of the airline. Wolf noted that United, too, was retiring older aircraft that consume relatively more fuel and replacing them with more efficient airplanes.

Delta ends its fiscal year in September. It reported net earnings for the year of $303 million, the third-highest in the company's history, but 34 percent less than what it earned in 1989. Again, the company blamed the downturn on higher jet fuel costs.

Delta Chairman Ronald W. Allen noted that airlines have taken steps including fare increases to offset the higher price of fuel, but added, "There is a limit to how much fares can be raised without depressing traffic unduly."

In stock market trading yesterday, UAL's stock closed at $96.50, down 37 1/2 cents; Delta closed at $58.62 1/2, down $3.12 1/2; and USAir closed at $16, down 62 1/2 cents.