USAir Inc., responding to an overall slump in the airline industry, yesterday announced plans to lay off another 2,100 employees in an effort to cut costs and stem its losses.

Announcement of the new layoffs came just 72 hours after the company said it was dismissing 1,500 probationary employees and scrapping its latest plans for expansion. A week earlier, the Arlington-based carrier announced it would lay off 211 pilots.

Yesterday's cutbacks bring to 3,600 the total number of people being dropped from the company's payroll of 54,000 employees.

About 5,700 of those people are in the Washington-Baltimore area, but company officials said it was impossible to tell how many area employees might be at risk from the layoffs.

"These additional personnel cuts take into account the softening of passenger demand, poor economic conditions, escalating fuel costs and the company's need to reduce operating costs," USAir Chairman Edwin I. Colodny said in a statement.

The problems facing USAir, to one degree or another, are facing all the major air carriers.

The industry has been particularly hard hit by soaring jet fuel costs brought on by the crisis in the Persian Gulf. But the air travel market has become so weak that the airlines were forced to rescind fare increases they had announced in an attempt to pass on those rising costs.

Colodny called the layoffs "regrettable," but said they were necessary to "more realistically realign our growth rate for the next two years with anticipated market conditions and reduce our operating costs."

Colodny predicted that the entire airline industry will experience a slump for at least two years.

The personnel reductions are not expected to affect the number of flights it operates or the cities it serves.

USAir has 2,971 daily flights, including nearly 100 daily flights from the Washington-Baltimore area. USAir Express, the airline's separate commuter service, is not involved in the layoffs.

The additional layoffs were not unexpected.

When the company announced Tuesday that it was dismissing the probationary employees it said it was conducting a study to determine if further layoffs were needed. He said the additional layoffs were the result of that study.

USAir has trimmed its passenger growth projections from 6 percent to "up to 2 percent" for 1991. The revised estimates reflect an overall slump in the airline market and a particularly sharp drop in the economically depressed areas of the Northeast and Middle Atlantic states, where USAir operations are primarily concentrated.

Because USAir is primarily a short-haul carrier, it is particularly vulnerable to the financial effects of a sudden rise in fuel costs, such as the airlines have experienced in the last month.

At the same time, competitive pressures have prevented the company from raising its prices to recoup those added costs.

Just this week, USAir was forced to put on hold a proposed 5.3 percent across the board increase in fares.

In the latest quarter, the company lost $75 million, its fourth strait quarter loss.

Robert Joedicke, an industry analyst for Shearson, Lehman, Hutton Inc., called the cutbacks "prudent management" and described the layoffs as a "sound move."

Joedicke said the cutbacks were not unexpected, noting that USAir was one of the first airlines to delay the delivery of aircraft. He said there was no question that USAir was sound financially, despite the cutbacks.

Paul Karos, an analyst for First Boston, agreed. "This is still a financially strong airline," he said, "they're not even in short-term trouble." He said there was no concern that the company might run short of cash.

Karos said he estimated that USAir had nearly $3 billion worth of aircraft assets that could be used either for lease-back arrangements or loan purposes in the event it needed cash.

Company officials said they did not expect anymore layoffs. "We hope this is the end of it if the economy doesn't worsen," one official said. Another agreed, but warned "you never say never."

To help ease the pain to its employees, company officials said they would layoff the 2,100 workers over the next 120 days to give employees a chance to consider new early retirement and "voluntary separation" programs that soon will be offered. The number of employees who eventually will be forced off the payroll will depend on how many people take the buyout plans. Details of the buyout incentives were still being worked on yesterday, a company spokesman said.

A breakdown of the total number of layoffs announced to date involves 211 pilots, 650 flight attendants, 2,100 reservations clerks and customer and fleet service agents as well as 500 management positions.

The announcement of the layoffs was made yesterday after the close of trading on the New York Stock exchange, where USAir shares closed up 12 1/2 cents at $14.25. For the week, however, USAir shares were down nearly 20 percent.