May 13--Southwest Airlines Co. has borrowed $600 million by mortgaging 21 of its Boeing 737 airplanes, the company disclosed Monday.

In the loan papers, the Dallas-based carrier said the money was to be used "for general corporate purposes."

A Southwest spokeswoman said the company simply wanted to boost its liquidity.

"We felt it was wise to bolster our cash position with today's soaring fuel prices and uncertainty in the economy and the credit market," Southwest spokeswoman Beth Harbin said. "We want to be prepared for whatever happens."

In a filing with the Securities and Exchange Commission, Southwest said it entered a term loan agreement May 6 with Citibank NA and seven European banks to borrow up to $600 million.

On Friday, it borrowed the entire amount, to be repaid by May 9, 2020.

Southwest earned $34 million in the first quarter, making it one of a handful of U.S. airlines and the only one of the nation's 10 largest carriers to post a profit.

Analysts expect the company to earn about $250 million for the full year.

However, Southwest chief executive Gary Kelly said the airline is struggling to boost revenue enough to stay ahead of rising fuel prices, even with its fuel hedges.