NEW YORK (CNN/Money) - Continental Airlines said it expects to post a significant first-quarter loss and warned that its expected return to profitability in the second quarter will be challenging.

Houston-based Continental, the nation's fifth-largest airline, did not give specific earnings or loss per share guidance for either period. But it said it is concerned with continued low fares, measured by revenue per mile flown by each available seat, as well as upward pressure on prices for fuel, which traditionally is an airline's second-largest cost after labor.

The airline said it managed to report a profit in March as miles traveled by paying passengers on Continental fell 5.4 percent from a year ago to 5.4 billion, but the airline's capacity decreased 10.4 percent. That resulted in a higher percentage of seats being filled. But despite fuller planes the revenue per available seat mile, known as RASM, was off 6 to 8 percent from a year earlier and off 11 percent from February.

"We are pleased that we achieved profitability in March but we continue to be concerned about higher fuel prices and RASM weakness due to capacity increases by many of our competitors," Chief Financial Officer Jeff Misner said.

Analysts surveyed by earnings tracker First Call had a consensus forecast of a loss of $2.07 a share in the first quarter, compared with a profit of 16 cents a share a year earlier, and a profit of 41 cents a share in the second quarter, down from 74 cents a year earlier. Continental is the only airline among the nation's six largest carriers that had been expected to make money in the second quarter.

Shares of Continental (CAL: Research, Estimates) lost 67 cents to $27.65 Monday before its statement.