McDonald's in Japan says it can only serve customers small portions of french fries as it deals with a shortage of its famous side order.

Industrial disputes on the United States' west coast are stifling exports of french fries, leaving Japanese McDonald's restaurants scrambling to secure fresh supplies.

The Japanese arm of the fast food giant said despite efforts to secure supplies, it will only serve french fries in small portions.

More than 1,000 tonnes of potatoes have been airlifted in and an emergency shipment of 1,600 tonnes is en route from the US east coast via an unusual sea route.

But the company said "this is not enough to offer a stable supply to customers" and for now it will only serve small portions of chips.

"We will temporarily change our product line-up ... until a stable supply of potatoes is in sight," it said in a statement.

"It is difficult for us to get potatoes in a sustainable manner due to lagging labour negotiations on the US west coast."

Dockworkers in the US were reportedly on a go-slow and were not providing full crews for months in a bid to gain bargaining leverage in labour negotiations with employers, hampering exports to Japan.

Earlier this month the operator of the Gusto restaurant chain said it planned to airlift in around 200 tonnes of french fries to avoid running short.

News of the chip dearth came as a butter shortage in Japan showed signs of easing.

For weeks, supermarket shelves were empty, with any new stock disappearing almost as fast as it arrived, despite store-imposed limits of one pack per customer.

The government stepped in and bought more butter on the international market, as well as encouraging domestic producers to up their output.

AFP