At least three illegal miners have been killed and 18 others trapped when a diamond mine collapsed in South Africa's Northern Cape province, local police said Wednesday.

The accident happened in an area formerly operated by diamond giants De Beers.

The company said it had stopped mining in the area two years ago, and that the illegal miners had ignored signs and barriers warning not to enter the disused shafts.

One man was rescued and he survived the incident. Three are confirmed dead so far, provincial police commissioner Janet Basson said, according to Reuters.

People are knowingly entering mine property, destroying fences, ignoring signage, evading arrest, transporting mining equipment and undertaking the dangerous excavation of tunnels which have proven to be unsafe and have in fact cost the lives of illegal diggers in the past, De Beers said in a statement.

The company added it had recently been warning local people about the dangers of entering the disused site.

South Africa is littered with disused mines, which are often worked by illegal diggers, many of whom are migrants from neighboring Zimbabwe or Mozambique.