DETROIT — Hyundai plans to hire 877 more workers to add a third shift at its Montgomery, Ala., assembly plant so it can crank out more Sonata midsize cars and Elantra compacts.

The Korean automaker said Monday the hiring will take place during the summer, with the plant transitioning to the third shift in September. The move will give Hyundai the ability to make 20,000 more vehicles at the plant in 2012.

The added shift will help get more cars to Hyundai dealers nationwide, who have been complaining for months that they could sell more cars if they could get them.

The Sonata and Elantra are Hyundai’s top-selling vehicles in the U.S.

Through March, Hyundai sold more than 55,000 Sonatas, with sales up more than 6 percent from last year. The company sold more than 44,000 Elantras during the same period, an increase of almost 8 percent.

The added shift will bring total employment at the plant to more than 3,000 people, Hyundai said.

The company also expects the additional production to create more jobs at companies that make parts for the two cars.

Hyundai has been struggling for months to produce enough cars to satisfy demand, trying to squeeze as much production as possible out of the Montgomery factory.