FRANKFURT (Reuters) - Thirteen people were injured, four of them seriously, in an explosion on Monday at a German factory operated by ball-bearings maker Schaeffler SHA_p.DE, German police said.

An explosion at a cleaning facility for industrial salts at Schaeffler’s plant in Eltmann, Bavaria, was reported to police in nearby Schweinfurt at around 1015 am local time (0815 GMT), according to an emailed statement.

More than 120 firefighters as well as police and a large contingent of rescue staff were on the scene but police said there appeared to be no danger for people living nearby as no hazardous or toxic substances were released.

The four people seriously injured were flown by helicopter to hospitals while workers at the factory were asked to briefly vacate their positions. The majority of the labor force have since returned to work, police said.

Criminal police in Schweinfurt and prosecutors have started investigating the incident.

Schaeffler, listed on Germany's mid-cap index MDAX, also makes components for transmissions and engines used by major carmakers Volkswagen VOWG_p.DE and Daimler.

Eltmann is located in Lower Franconia, a district of Germany’s southern state of Bavaria.