HONOLULU — Southwest Airlines has started a new route in the Hawaiian islands with plans to add more interisland service.

The airline began service between Honolulu on Oahu and Kona on Hawaii Island on Sunday.

Southwest plans to begin four-times daily service to Hilo on the eastern side of the Big Island in late 2019 or early 2020, officials said.

Southwest entered the Hawaii market in March when it initiated service between Honolulu and Oakland, California.

With the addition of Kona, Southwest has 16 Hawaii interisland flights, or 1,800 daily interisland air seats.

The Hilo service would add 1,400 more seats each day to the interisland market, where Southwest flies 175-seat Boeing 737-800 aircraft.

The Dallas-based carrier has added 2,100 daily seats through the 12 daily trans-Pacific flights that it offers to and from Hawaii, officials said.

Southwest also expects to announce upcoming Hawaii service from San Diego and Sacramento, California, officials said.