July 13 (Reuters) - United Airlines (UAL.O) said on Tuesday it would buy 100 19-seat ES-19 electric planes from Swedish start-up Heart Aerospace, as the U.S. carrier eyes battery-powered aircraft for regional routes. It is the latest in a series of such "innovation-related" announcements by United as major airlines come under pressure to cut emissions. The U.S. carrier"s venture funding arm said in a release it is also investing an undisclosed amount in the company with Bill Gates" Breakthrough Energy Ventures and Mesa Airlines (MESA.O). United would not disclose the value of the order, which is conditional on the aircraft"s meeting safety, business and operating requirements. Mesa would buy an additional 100 ES-19s, subject to similar requirements, which can fly customers up to 250 miles. The plane will enter service as soon as 2026. Electric planes can make only short urban or commuter trips because of the size and weight of the batteries. United could use the aircraft to connect hubs like Chicago O"Hare International Airport to smaller routes. Some of these routes have been cut by legacy carriers because they are too expensive. "With a lower cost profile you can bring back more smaller markets than were possible with the higher cost of a jet-fueled aircraft," Mike Leskinen, president of United Airlines Ventures, said in an interview. He sees the aircraft, which generate zero operational emissions, as a stepping stone toward larger planes. "We don"t want to wait for 50 seats, 75 seats, 125-seat aircraft," Leskinen said. "We want to get involved now by investing in a company that we think has a big technological lead with the hopes of over time working with them to move the size of the aircraft to larger gauge." This year, the carrier also invested and announced an order in electric aircraft company Archer. United has said it would reduce its greenhouse gas emissions by 100% by 2050.