"It was caused by the clandestine artisanal diggers who have infiltrated (the mine)," he told Reuters. "The old terraces gave way, causing significant amounts of material to fall."
"KOV is a delicate site and presents many risks," he added.
Glencore said in a statement that it had confirmed 19 fatalities so far and was assisting search and rescue operations by local authorities.
The statement said an average of 2,000 illegal miners sneak onto KCC property each day. The rudimentary and often outdated practices employed by artisanal miners can compromise the safety of the mines, and accidents among them are common.
Delphin Monga, provincial secretary of the UCDT union which represents KCC employees, said a crack in that part of the pit had been noticed on Wednesday. He said KCC had put up red warning signs, but the diggers had ignored them.
The KOV mine, which spans a vast flat expanse on the outskirts of the city of Kolwezi near the Zambian border, is one of the largest high-grade copper assets in the world.
The collapse of a 250-metre wall inside the same pit killed seven mine employees in 2016.
Thousands of illegal miners operate in and around mines in southern Congo, which produce more than half of the world's cobalt, a key component in electric car batteries.
Mine disasters in Africa have cost the lives of numerous miners, especially unauthorised artisanal miners who operate without safety standards or regulations.
At least nine illegal gold miners died in Zimbabwe when they were trapped in a mine last month.
Twenty-two died in a previous Zimbabwean gold-mine flood in February, and 14 tin miners were buried alive in Rwanda after heavy rains in January.
In February, about 20 people died when a truck carrying acid to Glencore's Mutanda Mine in DRC collided with two other vehicles.
Congo's military deployed hundreds of soldiers last week to protect a copper and cobalt mine owned by China Molybdenum Co Ltd from illegal miners.
Shares in Glencore closed down 4.9%, their worst day of trading since December. The company said the incident has not affected output.
BMO Capital Markets analyst Edward Sterck said if the incident is related to illegal mining, any impact may be relatively short-term beyond an investigative period.
"However, preventative action will likely be needed and it could impact Glencore's social licence to operate," he added.
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