Standard & Poor's placed Southwest ( LUV) on CreditWatch with negative implications after the carrier reported a first-quarter loss.

The placement "is based on the company's weaker-than-expected revenue and earnings outlook," S&P said, in a report late Thursday. "The company's financial profile will be affected also by increased levels of secured debt and leases arranged since late 2008 to maintain adequate cash balances," the ratings agency said.

Southwest reported Thursday that excluding special items, its first quarter loss was $20 million, its first quarterly loss excluding items since the first quarter of 1991. It said revenue per available seat mile fell 2.9% during the quarter and is unlikely to improve in the second quarter. While most airline shares rose on Thursday, Southwest shares fell 54 cents or 7% to close at $7.10.

Given the first quarter loss, the negative RASM outlook and increased debt, "the company's 2009 earnings and cash flow will likely come in below our previous expectations and its debt level above our previous expectations," S&P said.

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