Burger King (search) Thursday said sales at U.S. restaurants open at least a year in May posted their biggest gain in four-and-a-half years as efforts to revive the world's No. 2 burger chain gained traction.

The Miami-based company said same-store sales at U.S. franchised restaurants rose 7.2 percent during the month, while same-store sales at U.S. company-owned restaurants rose 10.1 percent. The increases are the largest since November 1999.

Over 90 percent of Burger King restaurants are franchised.

Burger King Chief Executive Brad Blum attributed the strong results to better-than-expected sales of new menu items such as its TenderCrisp fried chicken sandwiches (search) and Fire-Grilled salads, operational improvements, and a quirky new advertising campaign that has resonated with consumers.

"There is more passion in the restaurants among the employees and the franchisees," Blum, who joined Burger King as CEO at the beginning of last year, said in an interview. "There is a sense of increasing confidence that we're going to continue to build this kind of momentum."

Looking ahead, Blum said Burger King is focused intently on accelerating its sales momentum during the key summer months.

As part of that plan, the company has tied-in with the summer blockbuster film "Shrek 2" and will also begin advertising the most recent addition to its menu, the Angus Steak burger, with new television commercials later this month.

Burger King, which has been working to revitalize sales and find its footing in the competitive U.S. fast-food market, hired ad agency Crispin Porter + Bogusky (search) earlier this year after dumping WPP Group Plc's Young & Rubicam.

Burger King operates more than 11,200 restaurants in the U.S. and abroad, but trails McDonald's Corp. (MCD) in both sales and restaurants.

In the near term, Blum said its U.S. business will be focused on improving operations at its existing restaurants rather than opening large numbers of new stores.

But he said there is still a lot of room for growth overseas, particularly in Latin America and Asia.

"We have what we consider to be very high levels of potential internationally," Blum said, adding that the company also has opportunities to add stores to markets it is already in, such as Germany and the United Kingdom.

"It's undersaturated for us in a number of markets and the brand is well-received."

Private equity investors Texas Pacific Group, Bain Capital and Goldman Sachs Capital Partners bought Burger King from British drinks company Diageo Plc (search) in 2002.