McDonald's says a key sales figure dropped again in January as the world's biggest hamburger chain struggled with ongoing weakness in Japan and supply chain issues in China, The Associated Press reports.

The Oak Brook, Ill.-based company said global sales at restaurants open at least 13 months dropped 1.9 percent for the month. The figure is a key metric because it strips out the volatility of newly opened and closed locations.

In the U.S., the figure edged up 0.9 percent. But in Europe, the company's biggest market, it fell 2.1 percent. In the region encompassing Asia, the Middle East and Africa, it sank 9.5 percent.

It's the second decline in just four months for McDonald's Corp. The sales figure also dropped in October. It had been nearly a decade since its last monthly decline.