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 further details. Prosecutors could not immediately be reached for comment on the issue.

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 were also 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.

At the time, McDonald's said it was cooperating with an investigation by the South Korean government into the possible food contamination case.

Last year, talks to sell McDonald's South Korean business to domestic firm KG Chemical Corporation collapsed over "a large difference of opinion".