Morowali. A smelting furnace in a nickel processing plant in Central Sulawesi province exploded early on Sunday, killing at least 12 workers, officials said.

The affected plant is owned by Indonesia Tsingshan Stainless Steel (ITSS), a mining firm operating within the Indonesia Morowali Industrial Park.

The explosion occurred during repair work involving multiple workers around 5:30 a.m. Central Indonesian Time (9:30 p.m. Saturday GMT), said Dedy Kurniawan, a spokesperson for the industrial park.

"By 9:10 a.m., the fire had been fully extinguished," Dedy said in a statement.

Following the incident, the deceased and injured workers were promptly taken to the medical center located within the industrial park. Presently, at least 39 workers are under medical care, with some reported to be in critical condition.

Sources said that among the deceased, seven were local workers, while five were foreigners.

While the police have arrived at the scene to investigate, they have yet to issue an official statement.

"An investigation is ongoing to determine the cause of the incident. Victims with burn injuries have been transported to the medical center for urgent medical attention," said Abdul Hamid, a spokesperson for the Morowali District Police.

The process of nickel smelting is known to be hazardous due to the requirement of high smelting temperatures reaching up to 1,350 degrees Celsius to produce iron sulfide, commonly referred to as matte.

ITSS is a subsidiary of China-based Eternal Tsingshan Group.