DALLAS - Southwest Airlines said Monday it borrowed $600 million from Citibank and seven European banks to bolster its cash position in the face of economic uncertainty and soaring fuel prices.

The airline disclosed in a regulatory filing that it tapped the full amount of the loans on Friday. The loans were secured by the first-lien mortgages on 21 of Southwest's Boeing 737-700 aircraft. The filing did not give a reason for the loans, but spokeswoman Beth Harbin cited uncertainty about the economy, tight U.S. credit markets and the high price of fuel.