By Anait Miridzhanian and Olivia Kumwenda-Mtambo

JOHANNESBURG (Reuters) -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 an illegal mining operation.

Gauteng Province Premier Panyaza Lesufi said investigations were under way to determine how the cylinder in which the gas was stored sprung a leak overnight, and what type of gas was involved.

On Thursday he visited the site of the disaster near Boksburg, east of Johannesburg. In December, a gas tanker explosion in the same township killed dozens and destroyed houses and vehicles.

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

"Whether the (suspected) illegal miners are among the deceased, that is not yet known," William Ntladi told broadcaster SABC, which gave no further details.

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

Lesufi said bodies of the victims of Wednesday's leak were scattered nearby, with the youngest a one-year-old child.

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

Lesufi told reporters on Thursday he shared local residents' 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 the firepower of these illegal miners," he said.

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 combed the area on Wednesday night together. Those teams will continue their investigations on Thursday 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," he said.

"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.

(Additional reporting by Siphiwe Sibeko, Shafiek Tassiem; Writing by Bhargav Acharya; Editing by John Stonestreet)