The Whopper is striking back in the burger wars.

Burger King on Thursday reported a sharp uptick in its U.S. business, and executives said that work to revamp its signature burger earlier this year helped deliver an 8.5% increase in domestic U.S. same-store sales for the three months ended June 30.

"A lot of people are saying they're coming back for the first time in a long time," said Tom Curtis, president of the chain's U.S. and Canada business, in an interview.

The chain is gathering momentum while archrival McDonald's is grappling with challenges in its U.S. business, with traffic declining after recent promotional offerings and marketing failed to hit. McDonald's is bringing in a new U.S. president to conduct a turnaround and implement a new strategy focused on improving food and service.

Curtis said that Burger King was capturing share from competitors, and thinks some of it is coming from McDonald's. He said he intended to convert new customers into lifetime fans.

"The next generation of burger lovers are being exposed to Burger King, and that means we've got runway ahead for years to come," he said.

McDonald's is working to upgrade its own menu, step up customer service and develop new store prototypes. The chain is testing new hand-breaded chicken offerings and a line of brightly-colored beverages, including new drinks that include Red Bull.

"We've got to elevate the taste and quality of the food," said McDonald's Chief Executive Chris Kempczinski on an investor call this week. "We're going to elevate the experience that we're offering our customers."

Burger chains have been working to elevate their signature sandwiches, with both McDonald's and Burger King staging overhauls in recent years. McDonald's in 2023 debuted a renovation of its Big Mac and other core burgers in the U.S., promising hotter and juicier sandwiches.

Burger King, owned by Restaurant Brands International, earlier this year improved the bun, box, mayo and other elements of its Whopper.

Curtis said that the Whopper upgrades were working, and the chain now needs to focus on hospitality. The U.S. Burger King chief said he was still taking direct calls from customers on his phone several days a week, and friendly service comes up as one of the top issues.

The company in July said it would start guaranteeing the quality of its Whoppers, committing to remake orders on the spot if customers deem them unsatisfactory, and providing a future Whopper for free.

"I have a strong belief that we will ultimately someday be the standout burger chain in hospitality and friendliness," he said.