Shares of U.S. airlines tumbled Thursday after Delta Air Lines forecast slightly lower revenue growth, following downbeat sales projections from iPhone-maker Apple that stoked concerns of a global slowdown.

Delta shares fell nearly 9 percent, their steepest percentage decline in more than six years, to close at $45.61. American Airlines shed more than 7 percent, recovering slightly after hitting its lowest price since July 2016. United Airlines lost 5 percent, while Southwest Airlines ended the session more than 3 percent lower.

Delta warned investors before the market opened that its unit revenues in the last three months of the year, a gauge of airfare, likely rose 3 percent, down from a previous forecast of 3.5 percent. The Atlanta-based carrier said yield growth from last-minute bookings "was more modest than anticipated" in December.

The selloff presents airline executives with a tough crowd to please when they start reporting earnings this month. Delta is expected to report in the middle of the month. Investors have been skeptical of the sector, even though it's heading for its ninth year in a row of profitability. Spirit and United were the only two U.S. airlines whose shares gained in 2018.