United Airlines will add service to Paine Field-Snohomish County airport near Seattle in 2018, joining Alaska Airlines in returning commercial air service to the airport.

The Chicago-based Star Alliance carrier will offer six daily flights to Denver and San Francisco from the airport, which is 37km north of downtown Seattle, from August 2018, it says in a letter to employees on 10 August.

"Bringing new service to Paine Field offers customers more ways to conveniently connect to the country's largest business and leisure destinations," says Dave Hilfman, senior vice-president of worldwide sales at United, in a statement. "North Seattle and Northwest Washington area travellers will now have easy access to our hubs… and to opportunities to connect to business centres around the world with just one stop."

The mainline carrier will join Alaska at the airport, where a new two-gate passenger terminal is under construction by private operator Propeller Airports. Alaska has agreed to be the first commercial carrier at Paine Field with nine daily flights from the fall of 2018.

Seattle-based Alaska has not disclosed where it will fly from Paine.

Paine Field was the Seattle area's first commercial airport before it was replaced by Seattle Tacoma International airport in 1946.

United's new flights to Paine will complement the 36 it offers to Seattle Tacoma, it says.

Updated with August 2018 launch date

Source: Cirium Dashboard