The death toll from a leak of poisonous gas in a South African shantytown rose to 17 on Thursday, as officials suggested the accident was probably linked to illegal mining.

Gauteng Province Premier Panyaza Lesufi, visiting the site of the disaster near Boksburg, east of Johannesburg, said investigations were under way to determine how the cylinder in which the unidentified toxic gas was stored had sprung a leak.

Initial investigations indicated the gas could be linked to illegal mining, a spokesperson for the Disaster and Emergency Management Services in Ekurhuleni municipality said.

South Africa has been plagued by illegal mining, mostly for gold or coal, for decades, robbing the sector and state coffers of billions of rand through smalltime pilfering as well as networks run by organised crime.

"Whether the (suspected) illegal miners are among the deceased, that is not yet known," William Ntladi told broadcaster SABC.

Grieving locals described family members dying while trying to escape the scene of Wednesday's gas leak.

"I bumped into my cousin walking and crying and I asked him what's wrong. He told me that all his kids had died," said Felsa Nhamussa, who also lost her brother-in-law.

"When I woke up this morning, I came back to check what the situation is like, they told me that my brother-in-law was running away trying to escape and he fell and died."

Lesufi described the scene as "heartbreaking", with the bodies of the victims who included a one-year-old child scattered nearby. He said the death toll had reached 17, with four people still critically ill in hospital.

'OUT OF CONTROL'
Lesufi told reporters he shared local frustrations when told they were aware of 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... these illegal miners," he said.

Preliminary investigations showed the toxic gas was used by illegal miners to extract what they thought to be gold from the soil, the provincial government said in a statement.

Cyril Ramaphosa's office said the South African president had urged investigators to find the cause of the accident to avoid similar disasters in future.

A clip shared by Lesufi on social media showed several cylinders mounted on top of wooden tables in a shack covered with iron sheets. He shared an image of another cylinder, citing it as the source of the leak, without providing evidence.

Forensic workers in hazmat suits who combed the area on Wednesday night will continue their investigations and try to secure the area, Lesufi said.

"They've tried to ensure that those cylinders that are still there cannot either explode or they cannot harm people further. When I came here last night the smell was still up in the sky."

In May, a methane gas explosion in a disused South African mine killed at least 31 people believed to be from neighbouring Lesotho, while a gas tanker explosion in December killed dozens of people in the same township as Wednesday's deadly leak.