At least 17 people including three children have been killed by a leak of toxic nitrate gas being used by illegal miners to process gold in a settlement in South Africa.

Gauteng Province Premier Panyaza Lesufi visited the Angelo settlement in Boksburg, a city on the eastern outskirts of Johannesburg. 

“The scene was heartbreaking,” he said on Thursday, giving an updated death toll of 17, with four others critically ill in hospital.

Police said the three children killed were aged one, six and 15.

Lesufi told reporters he shared local residents’ frustrations about illegal mining operations.

“This thing of illegal mining is completely out of control … we really need our police force to be given the necessary firepower to match the firepower of these illegal miners,” he said.

Boksburg is the city where 41 people died after a truck carrying liquefied petroleum gas got stuck under a bridge and exploded on Christmas Eve.

Emergency services spokesman William Ntladi said Wednesday’s deaths were caused by a nitrate gas that leaked from a cylinder being kept in a shack. 

He said indications were the cylinder was being used by illegal miners to separate gold from dirt and rock.

Authorities did not say if the illegal miners believed to be responsible for the gas leak were among the casualties.

Illegal mining is rife in the gold-rich areas around Johannesburg, where miners go into closed-off and disused mines to search for any deposits left over.

Mining fatalities underground are also common and the South African government department responsible for mining announced recently that at least 31 illegal miners were believed to have died in a gas explosion in a disused mine in the city of Welkom in central South Africa. 

The cause was methane gas, the mining department said.

Wednesday’s tragedy was likely to stoke more anger at illegal miners, who are often migrants from neighbouring countries, operate in organised gangs and are blamed for bringing crime into neighbourhoods.

By Mogomotsi Magome and Gerald Imray in Johannesburg with Reuters