Major U.S. airlines on Wednesday said revenues were still improving, but discount carrier JetBlue Airways pointed to weaker-than-expected holiday sales, sending its shares down more than 8 percent.

JetBlue cut its fourth-quarter revenue outlook, saying higher yields had not materialized as it had planned.

We had planned on the peaks being slightly stronger than maybe what the peaks are turning out to be, JetBlue Chief Financial Officer Ed Barnes said during a presentation at the Hudson Securities airline conference.

JetBlue cut its fourth-quarter forecasts for two important performance measures -- passenger revenue per available seat mile and revenue per available seat mile.

The company forecast a rise of 10 percent to 12 percent in passenger revenue per available seat mile, compared with a prior view of 12 percent to 15 percent. It said revenue per available seat mile would probably rise 8 percent to 10 percent, down from the previous view of 10 percent to 13 percent.

Two of JetBlue's bigger rivals, Delta Air Lines and American Airlines parent AMR Corp , said they were not seeing weakness during the peak holiday period.

Delta Chief Financial Officer Hank Halter said the second-biggest airline expected to report a profit in the fourth quarter.

Bella Goren, chief financial officer of AMR, said advance bookings were holding up.

JetBlue shares were down 8.4 percent at $6.38 in morning trading, while Delta fell 0.5 percent to $13.06 and AMR declined 3.4 percent to $7.71. Industry leader United Continental Holdings was off 1.6 percent at $25.36.

(Reporting by Karen Jacobs in Atlanta; additional reporting by Lynn Adler in New York; Editing by Lisa Von Ahn)