At least 17 people were killed in an overnight gas cylinder leak in a South African shantytown on Thursday.

Gauteng Province Premier Panyaza Lesufi said investigations were underway to determine how the leak happened and what type of gas was involved.

Local officials initially blamed the accident on an illegal mining operation that went wrong.

On Thursday morning 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.

Bodies of the victims of Wednesday's leak were scattered over the area, Lesufi said, with the youngest a 1-year-old child.

"The scene was heartbreaking," he said, adding that one person had died in a hospital overnight, raising the death toll to 17, while four others were in critical condition.

A spokesperson for the Disaster and Emergency Management Services in Ekurhuleni municipality, where the disaster occurred, linked it to illegal mining.

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

Reuters could not immediately confirm Ntladi's comments.

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."