At least three illegal miners were killed and 18 others trapped underground when a tunnel collapsed in a disused diamond mine in South Africa's Northern Cape province, police said on Wednesday.
One man was rescued and he survived the incident. Three are confirmed dead so far, provincial police commissioner Janet Basson said.
The national rescue team was on the way to help the local authorities and police in their search for the other miners.
Diamond giant De Beers said the incident occurred in an area where it had stopped mining two years ago, adding that the miners had ignored warnings not to enter the disused shafts.
De Beers officials have been at the site since Tuesday.
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, it said in a statement.
Unions could not immediately be reached for comment.
The area is on the edge of Namaqualand Mines, which De Beers is in the process of selling to Johannesburg-listed miner Trans Hex.
De Beers said it had recently been telling people living nearby about the dangers of illegal mining via the regional radio and in community discussions.
South Africa has the world's deepest gold mines and is also a major producer of platinum and coal. Old or abandoned shafts are often worked by informal diggers, many of them migrants from Zimbabwe or Mozambique.
In a separate incident at a disused gold mine at Welkom in the Free State province, police said five men had been brought out alive since late on Tuesday but others remained trapped underground.
There are still people underground, alive. We cannot confirm how many because they are scattered, provincial police spokesman Sam Makhele said. The problem is that they can only be rescued one by one and that takes time.
In March, up to 20 illegal miners were thought to have died after a rock fall at another abandoned mine.