Impala Platinum says four workers have been killed in an underground fire at its Rustenburg mine in South Africa.
The fire began on Friday at Impala's Rustenburg 14 shaft and all but the four employees were evacuated by rescue teams.
The company on Sunday said those who died had been overcome by fumes while trying to find their way to safety.
South Africa's mines are the deepest and among the most dangerous in the world, but safety records in the industry have improved in the past few years.
The National Union of Mineworkers (NUM) said in a statement that Impala should improve health and safety conditions, and called on the mines ministry to investigate the cause of the accident.
"It is unacceptable that mine workers are denied their basic human right to work in an environment that guarantees their safety and that instead they are expected to go to work to die," said NUM health and safety secretary Erick Gcilitshana.
"We are selling our labour for the survival of our families, not our limbs and lives."
Mining Minister Mosebenzi Zwane was to visit Impala's Rustenburg shaft on Sunday.
Mining companies are required to halt operations after fatalities, according to South African law.