Southwest Airlines opens interisland service to Hilo today and marks a new milestone with the start of direct trans-Pacific service between the neighbor islands and the mainland.

The carrier today begins four daily interisland flights between Hilo and Honolulu, and adds a daily interisland flight between Kona and Kahului.

These flights are expected to mostly complete Southwest’s interisland expansion; although, demand could spark incremental growth. However, Southwest is expected to keep trans-Pacific competition intense as it offers more routes that compete with its key Hawaii competitors Hawaiian Airlines and Alaska Airlines.

Southwest will break into new territory today with the launch of its first nonstop daily between the neighbor islands and the mainland. The daily flight will go between Oakland and Kona on Mondays, Wednesdays, Fridays and Sundays; and San Jose and Kona on Tuesdays, Thursdays and Saturdays.

Southwest is also adding a daily flight between Lihue and Oakland on Tuesdays, Thursdays and Saturdays; and San Jose on Mondays, Wednesdays, Fridays and Sundays.

The carrier, which marks the first anniversary of its Hawaii service March 17, will begin offering a daily flight between San Diego and Kahului on April 14 and from San Diego to Honolulu on April 20. Those flights will bring Southwest’s Hawaii service to 14 flights daily from Hawaii to California and 38 departures a day on interisland routes.