A McDonald's outlet.

Oak Brook, Illinois - McDonald's says a key sales metric slipped again in the US as it faced “ongoing broad-based challenges” in May.

The world's largest hamburger chain said US sales declined 1 percent at locations open at least 14 months.

The fast-food chain has been struggling to boost sales amid heightened competition and changing eating habits.

any of its core customers are also struggling financially, which has forced McDonald's to intensify its focus on value.

Executives at McDonald's have also conceded that the chain introduced too many items too quickly last year, which led to slower service and inaccurate orders.

Chief executive Don Thompson has said the company is working with franchisees to address those problems, in part by ensuring restaurants have appropriate staffing.

The decline in the US comes after a 1.7 percent drop for the first three months of the year.

And last year, sales at established US locations slipped 0.2 percent.

Overseas, a rebound in China after last year's worries about avian flu helped lift sales by 2.5 percent in its Asia, Middle East and Africa division.

That in turn lifted global sales 0.9 percent for May.

In Europe, McDonald's largest market, the sales figure rose 0.4 percent, boosted by improvements in the United Kingdom and France.

An expanded beverage business was among the factors behind the improvements.

McDonald's, based in Oak Brook, Illinois, has more than 35,000 locations around the world.

Its shares slipped 19 cents to $101.77 in premarket trading Monday.

Its shares are up more than 5 percent so far this year. - Sapa-AP