Several major airlines are raising their ticket prices.

Southwest began the move by saying it will raise its ticket prices by $10 per roundtrip ticket on 10 percent of its routes.

American, Delta, U.S. Airways, JetBlue and Virgin America all followed suit. United took the move a step further, announcing its increase will cover the majority of domestic flights.

U.S. airlines have implemented four broad fare increases this year, about half as many as last year. None have raised rates in about a month.

Delta tried to lead an airline price hike last week, but failed.

The move is seen by some as a sign that airlines could be worried about filling seats in the current economy.