At least 20 employees of mining giant De Beers have been killed in a road accident in South Africa, local officials said Sunday.

The bus involved was ferrying staff from the Venetia mine, one of the biggest diamond mines in the country, said a transport official in Limpopo province, in the north of the country.

“The bus came into collision with a lorry,” said Vongani Chauke.

Chauke told AFP the accident happened at around 1600 GMT, some 25 kilometres (15 miles) from the mine, at the village of Musian on the border with Zimbabwe.

The cause of the accident has not been established.

South Africa has one of the most developed road networks on the continent, but also has one of the worst road safety records.

The Venetia mine, which lies near the borders with Botswana and Zimbabwe, has been run by the De Beers group for more than 30 years.

It accounts for more thatn 40 percent of the country’s annual diamond production, employing more than 4,300 staff including many local people.

It was once the country’s largest open-cast mine, before De Beers invested $2 billion in a major underground project to access the less easily available diamonds . The group aims to produce four million carats a year.

In July, De Beers announced the start of underground diamond production from the new seams opened underneath the open-cast mine.