Wendy's International Inc. (WEN), which has been struggling to reverse sluggish sales at its hamburger restaurants, Monday said its president and chief operating officer resigned, effective immediately.

Tom Mueller, who joined the No. 3 U.S. burger chain in 1998 following a long career with Burger King Corp. (search) , is leaving for personal reasons, Wendy's spokesman Denny Lynch said.

Mueller had overseen the Wendy's brand in the United States and internationally since becoming president in 2000. Lynch said he did not know where Mueller was headed next.

Wendy's Chairman and Chief Executive Jack Schuessler will assume the president's role on an interim basis, the Dublin, Ohio-based company said in a statement.

Wendy's has been working to keep up with larger rivals McDonald's Corp. (MCD) and Burger King, which have both improved their U.S. sales thanks to popular new products like meal-sized salads and chicken strips that Wendy's was once alone in offering.

As part of its plan to revitalize the Wendy's brand, the company said last month it would close up to 60 underperforming restaurants, slow new store development, and sell more U.S. outlets to franchisees.

Wendy's stock was down $1.03, or 2.2 percent, at $46.29 Monday on the New York Stock Exchange.