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 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.

After years of outperforming rivals, McDonald's has been struggling amid intensifying competition and challenging economic conditions around the world. Late last year, the company ousted the head of its U.S. business after the sales figure dropped for the first time in nearly a decade. CEO Don Thompson, who took the top spot this summer, has vowed to add business by focusing on value while planning a series of new limited-time offers to attract customers.

In the region encompassing Asia, the Middle East and Africa, however, McDonald's said its sales sank 9.5 percent in January. Like Yum Brands, which owns KFC, Pizza Hut and Taco Bell, McDonald's is dealing with wariness among diners after reports on national Chinese television that chicken producers were ignoring regulators and giving the birds unapproved levels of antibiotics.

In Europe, the company's biggest market, the figure fell 2.1 percent. Germany and France dragged down results. The company said it remains focused on enhanced value and keeping stores open longer in the region.

In the U.S., McDonald's said the figure edged up 0.9 percent, helped by the addition of the Grilled Onion Cheddar burger to the Dollar Menu.

Copyright 2013 Associated Press. All rights reserved. This material may not be published, broadcast, rewritten, or redistributed.