It was the latest in a series of deadly incidents at nickel smelting plants in Indonesia that are part of China's ambitious transnational development programme known as the Belt and Road Initiative.

Nickel is a key component in global battery production for electric vehicles.

At least four Chinese and nine Indonesian workers died when the furnace exploded while they were repairing it, said Central Sulawesi police chief Agus Nugroho.

The blast was so powerful it demolished the furnace and damaged parts of the side walls of the building, said Nugroho, adding that about 46 workers were injured, including 14 Chinese nationals, some in critical condition.

Authorities are working to determine whether negligence by the company led to the deaths, Nugroho said.

The accident occurred at PT Indonesia Tsingshan Stainless Steel, a subsidiary of PT Indonesia Morowali Industrial Park, known as PT IMIP, in the Bahodopi neighbourhood of Morowali regency.

“We sincerely apologise for this incident and we are working closely with authorities to investigate what caused the accident,” said company spokesperson Deddy Kurniawan.

Rescuers extinguished the fire and evacuated workers after a nearly four-hour operation, he added.

In a statement released on Sunday afternoon by the company, Kurniawan said the furnace was under maintenance and not operating at the time. However, “residual slag in the furnace" came in contact “with flammable items” driving the furnace walls to collapse and the remaining steel slag to flow out.

Previously, the company said explosive liquids at the bottom of the furnace triggered a fire and a subsequent explosion in nearby oxygen cylinders.

It was the third deadly incident this year at Chinese-owned nickel smelting plants in Central Sulawesi province, which has the largest nickel reserves in Indonesia.

Two dump truck operators were killed when they were engulfed by a wall of black sludge-like material following the collapse of a nickel waste disposal site in April.

In January, two workers, including a Chinese national, were killed in riots that involved workers and security guards at an Indonesia-China joint venture in North Morowali regency.

Last year, a loader truck ran over and killed a Chinese worker while he was repairing a road in PT IMIP's mining area, and an Indonesian man burned to death when a furnace in the company's factory exploded.

Nearly 50 per cent of PT IMIP's shares are owned by a Chinese holding company, and the rest are owned by two Indonesian companies. It began smelter operations in 2013 and is now the largest nickel-based industrial area in Indonesia.

Three Chinese workers in March filed a complaint to Indonesia's National Commission on Human Rights, alleging that their health is deteriorating due to dust and smoke exposure while working seven-day weeks without a break at PT IMIP. They added that workers there don't have adequate safety equipment.

Data collected by the Mining Advocacy Network, an Indonesian watchdog, showed that at least 22 workers from China and Indonesia have died in nickel smelting plants in Central Sulawesi province since 2019, including two Chinese nationals who committed suicide.