CHICAGO — Fast-casual chains continue to lead restaurant growth in the United States, according to Technomic, Inc., a Chicago-based market research firm. The segment posted an 11% increase in sales and nearly an 8% increase in units in 2013.

With better-burger concepts, Mexican chains, bakery-cafes and sandwich shops on the rise, the fast-casual landscape continues to become more diverse.

Among Technomic’s 2014 Top 500 Chain Restaurants, category leaders showed healthy growth last year, with Panera Bread’s sales up 12%, Chipotle Mexican Grill growing 17% and Panda Express increasing 11%.

Offering customized selections, quality ingredients and hip ambiance, the top growing fast-casual chains, led by BurgerFi and Hot Head Burritos, aim to further shake up the scene.

“Watch for even more menu and concept diversity in this sector,” said Darren Tristano, executive vice-president of Technomic. “Also look for more quick-service restaurants and full-service restaurants to realign their formats and develop new fast-casual concepts to compete more aggressively.”

Indeed, fast-food and casual-dining brands alike have tweaked menus and formats in an effort to match the success of the fast-casual segment. Such full-service restaurants as Applebee’s Neighborhood Grill and Bar and Red Lobster have tested express-lunch options, while quick-service chains like Wendy’s have pushed more premium offerings.

Yum! Brands, parent company of Taco Bell, Pizza Hut and KFC, in the past year has tested several fast-casual concepts, including Super Chix, a restaurant in Arlington, Texas, that features customizable sandwiches with a hand-breaded, peanut oil-fried chicken breast filet and such optional free toppings as jalapeños, hot pickles and haystack onions, plus several sauces, including honey mustard, smoky honey pepper, and sriracha sweet and sour. Last August, Yum! opened KFC Eleven, an upmarket version of its fast-food chicken chain, with a menu of updated side dishes, salads, rice bowls, flatbread sandwiches and only boneless pieces of Original Recipe chicken.

Yum! this year also introduced by-the-slice service in two Pizza Hut locations. At $2 to $3 a slice, the pizzas feature new recipes and are cooked in new deck ovens. One test location includes sautéed pasta and made-to-order salad stations, as well as an open-seating environment, a departure from the company’s traditional dine-in stores.

And Taco Bell, which introduced its Cantina Bell menu to mimic the build-your-own options at popular Mexican grills, is testing a fast-casual concept called U.S. Taco Co., which will feature such upscale items as tacos with lobster or brisket.

Click for a slideshow of top growing fast-casual chains.