McDonald's Corp. on Friday reported its worst monthly calendar-adjusted global same-store sales since early 2003 as it deals with a meat-supplier scandal in China and continued weakness in the U.S.

Global sales fell 2.5% in July, far worse than expected. The steepest drop came in its Asia/Pacific, Middle East and Africa region, where sales at existing locations slumped 7.3% last month. Analysts expected a 0.5% drop in that region.

The bigger problem for the company is continued weakness in the U.S., its largest market in terms of restaurants. For July, McDonald's reported the ninth consecutive month of negative or flat same-store sales growth in the U.S.—its longest stretch without growth in its core market since 2003.

The world's biggest restaurant chain and its chief executive of the past two years, Don Thompson, are currently wrestling with significant challenges on multiple fronts. McDonald's has lost sales momentum over the past year in its home market as U.S. consumers defect to fast food rivals such as Burger King Worldwide Inc. or fast casual chains like Chipotle Mexican Grill Inc.

Analysts are beginning to wonder how McDonald's will get the chain growing again. "While we admire McDonald's global footprint, strong brand, and proven business model, without clear comp drivers to act as catalysts, we remain unconvinced of a substantial reacceleration in results," Bernstein analyst Sara Senatore said.