A major Chinese manufacturer for global technology companies says it hired underage workers, some as young as 14.

Foxconn Technology Group said Tuesday that it employed an undisclosed number of underage interns for about three weeks at its Yantai campus in the Shandong province. The company said it took “immediate steps” to return the interns to their schools, but gave no other details about their employment. China's minimum legal working age is 16.

Foxconn is a major contractor for U.S.-based technology giant Apple and other retail technology companies. Foxconn said Apple products are not produced at the Yantai facility, but did not say what is made there.

Foxconn is owned by Taiwan's Hon Hai Precision Industry Company and is one of China's largest employers, with about 1.2 million workers.

The employment of underage workers is the latest in a string of workplace problems for the company. In recent weeks, production-line employees have protested their working conditions, while labor groups say the company has permitted 60-hour work weeks and violates health and safety rules.