"McDonald's China full menu will come back gradually by cities," the company said in an e-mailed statement. "We have been working to bring back the full menu across China, but it takes a longer time for good reasons."

"Suppliers must now increase their production capacity and re-allocate nationwide supply distribution," McDonald's said. "For new potential suppliers, we have a full due diligence inspection process."

OSI has since pulled all products made by Shanghai Husi and replaced its management team in the country after coming under Chinese government investigation. Six employees of Shanghai Husi have been detained by the Shanghai branch of the Public Security Bureau, Alison Kovaleski, a spokeswoman for OSI, said.

"We are also in the midst of conducting our own thorough reviews of all of our manufacturing facilities within China," she said. "We look forward to sharing our findings in a responsible and transparent manner."

In Hong Kong, McDonald's restaurants are serving burgers such as the Big Mac and McChicken sandwiches "back to their original build", after lettuce and onion were imported from the US and Taiwan, the restaurant chain said on its website. McDonald's Hong Kong halted sales of products supplied by Shanghai Husi late last month.

Chicken McNuggets and items such as fresh corn cups, green salad and fresh lemon tea, made with ingredients formerly sourced from OSI's Chinese units in Hebei and Guangzhou, are still unavailable, according to the Hong Kong chain.

McDonald's China outlets started handing out vouchers for free hash browns and french fries as a compensation to customers who buy its burgers made without fresh vegetables, the restaurant chain said on Tuesday.

It is increasing orders from existing suppliers such as McKey Food Services Ltd, a Shenzhen unit of US-based Keystone Foods, to handle the shortage of ingredients, while exploring new Chinese suppliers for items such as fresh produce, according to the company.

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