Following complains of children falling ill, investigators have raided the Seoul office of McDonald’s Korea.

It is alleged the youngsters had eaten undercooked hamburger patties, reports Yonhap News Agency.

Seoul central district prosecutors’ office has seized documents and evidence from McDonald’s as well as the offices of an ingredient supplier.

“We take this matter very seriously,” says McDonald’s spokeswoman Karen Kim. “McDonald’s Korea continues to fully co-operate with all relevant authorities.”

A consumer filed a complaint against the US fast-food giant in July saying her four-year-old daughter had been diagnosed with hemolytic uremic syndrome (kidney damage), often referred to as “hamburger disease”, after she had eaten a McDonald’s hamburger last year.

Complaints were also filed by the parents of four more children who became sick after eating McDonald’s burgers.

In August, customers of a McDonald’s outlet in the southwestern city of Jeonju reported stomach aches and high fever after eating bulgogi burgers. The chain temporarily halted sales of the product.

Talks to sell McDonald’s South Korean business to domestic firm KG Chemical Corporation collapsed last year over what KG called “a large difference of opinion”.