DALLAS — Southwest Airlines is further pressing its Pennsylvania assault on embattled US Airways with plans to begin flying to the bankrupt carrier’s former Pittsburgh hub this spring.

Analysts said the new flights, planned for May, could speed the demise of US Airways Group, which remains the dominant carrier in Pittsburgh.

The nation’s seventh-largest airline, which has been crippled by fare wars with Southwest, has said that if it can’t reduce labor costs it could begin liquidating assets this month.

A spokesman for US Airways, David Castelveter, said Wednesday the growth of low-cost service was not surprising in Pittsburgh “and further underscores the importance of us establishing a cost structure comparable to that of our low-cost competitors.”

Last May, Dallas-based Southwest moved into Philadelphia, one of US Airways’ hubs, and has since expanded service there, forcing US Airways to cut fares.

Southwest said traffic on Philadelphia-based routes to Chicago, Orlando, Tampa Bay, Phoenix, Las Vegas and Providence, R.I., rose 51%, and average fares fell 37%.

Southwest’s chief executive, Gary Kelly, said Southwest went into Philadelphia because it could undercut high fares there and became interested in Pittsburgh as US Airways reduced the number of flights. He denied targeting US Airways.

“It’s a coincidence,” he said during a conference call.

Mr. Kelly said Southwest would announce its Pittsburgh schedule and fares before April. He said Southwest would begin with fewer than five gates and a “modest” number of daily flights. The airline typically begins in new cities with one or two gates and 10 to 15 flights.

Industry experts said Southwest’s move into Pittsburgh would have dire results for US Airways.

“That’s the nail in the coffin. It’s the end of US Airways,” said Michael Boyd, president of The Boyd Group, an aviation consulting firm in Colorado. “There is just no way an airline like Southwest is going to go into Pittsburgh unless it knows US Airways is through and it knows there’s going to be a huge gap there.”

An analyst with Argus Research Corporation, Suzanne Betts, said Southwest took customers away from US Airways in Philadelphia and could do the same in Pittsburgh with lower fares and better service — especially after US Airway’s well-publicized mishandling of thousands of bags over Christmas because of bad weather and labor troubles.