A small fries is seen at a McDonald's restaurant in Tokyo, Wednesday, Dec. 17, 2014. McDonald's in Japan has begun limiting the serving size of fries as stocks of spuds run short due to labor disruptions on the U.S. West Coast. (AP Photo/Eugene Hoshiko)

TOKYO - Only small fries with that? McDonald's in Japan is limiting the serving size of fries as stocks of spuds run short due to labour disruptions on the U.S. West Coast.

McDonald's began rationing its fries Wednesday morning. It said prolonged labour negotiations with port workers on the West Coast have made it difficult to meet demand despite an emergency airlift of 1,000 tons of spuds and an extra shipment from the U.S. East Coast by sea.

Frozen french fries — ready for the deep-fryer — are a leading U.S. export.

Japanese consume more than 300,000 tons of french fries a year, mostly at fast-food restaurants, and largely made from imports of frozen, processed potatoes from America, according to U.S. figures. Shipments in December are expected to be just over half the normal level, Japanese newspapers reported.

But demand is rising as convenience stores are increasingly also selling fries.

McDonald's has 3,100 outlets in Japan. It cut prices for set meals to compensate for including only small fries.

Customers expressed disappointment as they left a downtown Tokyo McDonald's outlet on Wednesday.

"The kids like the bigger sizes, like M and L, so it's a shame," said businessman Kenichi Kuniki, 45.

Japan's locally grown potatoes are mostly eaten fresh, rather than as fries, and production has been declining for years. But Japan enforces strict limits on where and how fresh potatoes are imported.

The powerful dockworkers union and multinational shipping lines have been negotiating a new contract for about 20,000 West Coast workers. In the meantime, labour disruptions have slowed shipments and driven costs higher.

Japanese are also facing a shortage of butter that has prompted grocery stores to limit shoppers to one or two packages apiece. That shortage stems from declining domestic production plus trade barriers and other restrictions that limit imports.

The restrictions are meant to ensure that local farmers who face high costs here are protected from foreign competition, to ensure Japan maintains some self-sufficiency in its food supply, but supply doesn't always meet demand.

"It's a bit sad," said Hiroko Inomata, 34, clutching the bag of small fries and a teriyaki burger she bought for lunch. "But it is so that everyone can have some."

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Associated Press video journalists Kaori Hitomi and Emily Wang contributed.

Elaine Kurtenbach on Twitter: https://twitter.com/ekurtenbach

News from © The Associated Press, 2014