People practice at a fitness club located on top a McDonald's restaurant in Paris, France, September 29, 2017. REUTERS/Charles Platiau

SEOUL (Reuters) - South Korean investigators raided the Seoul office of McDonald’s Corp on Wednesday following a series of complaints that children fell ill after eating hamburger patties alleged to have been undercooked, Yonhap News Agency and other media said.

The Seoul central district prosecutors’ office confiscated some documents and evidence at the office and three other companies, including an ingredient supplier, the reports said.

A spokeswoman at the office confirmed the raid to Reuters but gave no reason or details. Prosecutors could not immediately be reached for comment on the issue.

“We take this matter very seriously,” company spokeswoman Karen Kim said. “McDonald’s Korea continues to fully cooperate with all relevant authorities.”

In July, a consumer filed a complaint against the U.S. firm, saying her 4-year-old daughter was diagnosed with hemolytic uremic syndrome, often referred to as hamburger disease, after suffering irreversible kidney damage following the consumption of a McDonald’s hamburger last year, Yonhap said.

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

In August, consumers at a McDonald’s in the southwestern city of Jeonju reported stomach aches and high fever after eating bulgogi burgers. The chain temporarily halted sales of the burgers, launched in 1997, to determine the cause.

Last year, talks to sell McDonald’s South Korean business to domestic firm KG Chemical Corporation collapsed over what KG called “a large difference of opinion.” [nS6N1BG01F]