The death toll after an explosion at a Chinese-funded nickel-processing plant in eastern Indonesia over the weekend has risen to 18 with dozens still being treated in hospital, a police official said Tuesday.

The island of Sulawesi is a hub for the mineral-rich country's production of nickel, a base metal used for electric vehicle batteries and stainless steel, and Beijing's growing investment has stoked unrest over working conditions at its facilities.

The accident happened on Saturday morning as workers did repairs on a furnace at a plant owned by PT Indonesia Tsingshan Stainless Steel in the Morowali Industrial Park in Central Sulawesi province.

"The number of victims who died increased by five people, so in total 18 people have died," Morowali police chief Suprianto, who like many Indonesians goes by one name, told AFP.

He said they died of their burns, with most of the victims suffering burns to more than 70 percent of their bodies.

Eight of the victims were foreign nationals and 10 were Indonesians, he said, adding 24 people were still being treated at the hospital. Six others were being treated at ITSS's clinics, bringing the total to 30 injured.

An initial probe showed the explosion happened when residual slag from the furnace, which was closed for maintenance, flowed out and came in contact with flammable materials around the location.

The fire was extinguished later on Saturday.

Tsingshan Holding Group, the world's biggest nickel producer and China's biggest stainless steelmaker, holds a majority stake in ITSS.

ITSS is a tenant in the industrial park, which is also majority owned by Tsingshan along with local partner Bintang Delapan.

A Chinese foreign ministry spokeswoman said on Monday Beijing had asked its embassy in Jakarta to investigate the blast in coordination with Indonesian authorities.

In January, two workers including a Chinese national were killed at a nickel smelting plant in the same industrial park after a riot broke out during a protest over safety conditions and pay.

In June a fire at the same plant left one dead and six others injured, in another incident that has caused concern about safety at facilities funded and operated by Chinese companies.