Glencore Plc said 19 illegal miners were killed when part of a mine collapsed in the Democratic Republic of Congo.

Glencore shares fell as much 8.3 per cent. The workers entered the mine without permission and put their lives at risk by trespassing on the industrial site, which is one of the world’s biggest cobalt mines, according to a statement from the company on Thursday. A search and rescue operation is ongoing. The incident will not affect Glencore’s production.

Producers and authorities in Congo and other African countries face a constant struggle against illegal miners, who break into operational and shuttered mines to extract ore and don’t follow official safety procedures.

A gold mine owned by AngloGold Ashanti Ltd. was forced to evacuate employees when it was overrun by thousands of informal miners in 2016. In South Africa, more than 30 illegal miners died in an explosion at an unused gold mine shaft in 2017.

Glencore operates the Komoto mine through its Katanga Mining Ltd. unit and the project is the company’s second-biggest source of cobalt. Sales of cobalt from the mine were halted last year due to low levels of radioactivity.

Reuters reported earlier that at least 36 people were killed in the collapse, citing Richard Muyej, the governor of Lualaba province. Eric Tshisola, chief of staff to Lualaba Mining Minister Jean-Marie Tshizainga, put the death toll at 23 so far.