Southwest Airlines flight 1117 from St. Louis lands at Boston Logan International Airport on March 13, 2019. (Photo by John Tlumacki/The Boston Globe via Getty Images)

A U.S. airstrike that killed Iran's top military leader drove up oil prices more than 3% on Friday and sent shares in airline stocks tumbling.

After labor, fuel is generally airlines' second largest expense, and carriers are particularly sensitive to volatility in the market.

All U.S. carriers were down more than the broader market.

American Airlines shares were down 4.8% in morning trading, while United and Delta were off more than 3%, and Southwest lost 2.4%.

Sustained higher crude prices could hurt airline balance sheets. Carriers are already facing higher labor costs as they negotiate with unions representing more than 120,000 employees this year and ramp up hiring.

U.S. airlines are scheduled to report earnings later this month.