McDonald's Corp posted a 2.2 percent rise in sales at existing restaurants worldwide, disappointing some investors and sending its shares down 2 percent in premarket trading.

Closely watched same-store sales rose 3.5 percent in Europe and 1.7 percent in the United States. They slipped 0.5 percent in the company's Asia Pacific, Middle East and Africa (APMEA) segment.

Jefferies & Co analyst Jeff Farmer expected same-store sales to rise 3 percent in the United States and Europe, and 2.3 percent overall.

The world's largest hamburger chain said its U.S. sales were supported by the new premium Angus Third Pounder burger and new McCafe coffee drinks.

McDonald's and some other fast-food restaurants have benefited as the global economic downturn has led diners to seek lower-priced fare, but long-term economic doldrums and a 9.7 percent U.S. unemployment rate have taken a bite out of profitable breakfast and beverage sales.

McDonald's said systemwide sales rose 1.1 percent in August, but were up 4.1 percent excluding the impact of currency fluctuations.

As people continue to spend cautiously, McDonald's has focused its advertising on its core and value menus and away from higher-priced food items -- with the exception of its new Angus burger, which is being introduced across the United States.

McDonald's shares fell 2 percent to $55.03 in premarket trading.

(Reporting by Martinne Geller; editing by John Wallace and Maureen Bavdek)