Three Zimbabweans were killed when a truck they were travelling in overturned on the N1 freeway near Richmond, Northern Cape police said on Tuesday.

Superintendent Hendrik Swart said six others were rushed to De Aar Hospital with serious injuries. Four people, including the truck driver and his assistant, were treated for minor injuries.

Swart said the truck, carrying 13 immigrants who were allegedly in the country illegally, was on its way from Bloemfontein to Cape Town when the driver lost control of it on Tuesday.

Police were investigating when and how the 13 got onto the truck.

The families of the victims had not been informed of their deaths yet.

Sapa