This is just continuing the expansion we resumed last year

Southwest spokeswoman

Meanwhile, investors showed their relief that United Airlines had struck a pay deal with mechanics unions, thus avoiding damaging strike action, by sending United's shares 19% higher in New York on Tuesday.

United lost at least $100m in sales as bookings tailed off in the two weeks before the agreement, according to Merill Lynch airline analyst Michael Linenberg.

Model airline

Southwest, which was the model for other budget airlines such as Easyjet and Ryanair, said the 4,000 new staff were needed to cope with its expansion plans and extra airport security.

"This is just continuing the expansion we resumed last year as well as adding people for new security", said Melanie Jones, a spokeswoman for Southwest.

Unlike most airlines, Southwest did not cut staff numbers or routes after 11 September, though it temporarily froze its expansion plans.

The new staff will include 250 extra pilots and 1,200 flight attendants. Most of the rest will be in customer services and reservations.

Profits halved

But Southwest has not been completely unscathed by the crisis afflicting the airline industry.

Profits fell more than half to $63.5m in the October-to-December period of last year compared to the final quarter of 2000.

Like many other major airlines, Southwest said then it could not accurately forecast for profitability in 2002 due to the weakness of the US economy and the travel slump.