American Airlines Group Inc. cut its forecast for second-quarter unit revenue and pretax margins, adding to concerns about profitability in the sector.

The largest U.S. airline by traffic now expects passenger revenue per available seat mile—an important measure of performance for the industry—will decline roughly 6% to 8%. American previously had projected a drop of between 4% to 6%. The carrier also again lowered its pretax margin view by one percentage point to between 16% and 18% for the quarter.

American also reported that its passenger traffic for May rose 0.7% from a year earlier. Capacity increased 2.1% while the percentage of seats filled—or load factor—declined to 82.8% from 84%.

Analysts and investors have been concerned that low fuel prices that are helping buoy profits in the sector could be outweighed by airlines oversupplying the market—a practice that has damaged the industry's profitability in past years.

On Monday, United Continental Holdings Inc. said its capacity in May rose 2.1% from a year earlier, and that it expects unit revenue in the current quarter to fall 5% to 6%, in part because of reduced travel spending by oil companies. Delta Air Lines Inc. last week said its unit revenue declined 5.5% in May from a year earlier.

American shares fell 1.7% to $39.20 in recent premarket trading.

Write to Tess Stynes at tess.stynes@wsj.com

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