At least 17 people have died, including children, after a gas leak at an informal mining town near Johannesburg in South Africa, officials said on Thursday.

"It's not a nice scene at all... It's painful, emotionally draining and tragic," said Gauteng province premier Panyaza Lesufi.

The incident was initially reported as a gas explosion but emergency services later discovered upon arrival that the problem was a leak from a cylinder containing a toxic gas.

A higher earlier death toll was revised downward after some victims were resuscitated.

What caused the gas leak?

Lesufi tweeted videos from a shack in the informal settlement of Angelo where at least four gas cylinders can be seen on metal stands.

The video also shows what Lesufi said was the cylinder responsible for the leak lying on the floor next to the entrance of the shack.

"The cause of the incident is alleged to be a nitrate oxide gas leakage from a cylinder used in an illegal mining activity in and around the settlement," said William Ntladi, a spokesperson for the emergency services.

"Apparently, the illegal miners used the gas to extract gold from the soil."

When rescue workers reached the scene, near the middle-class suburb of Boksburg, they encountered numerous individuals lying unconscious after inhaling toxic gas.

Angelo consists of shabby brick and corrugated iron sheet shacks and is at the base of an old, abandoned mine.

The dangers of illegal mining

South Africa, which has an unemployment rate exceeding 32%, has a significant number of illegal miners known as "zama zamas" (meaning "those who try their luck" in Zulu). These unregistered miners risk their lives scavenging for gold in defunct mines in harsh and hazardous conditions.

Johannesburg, the country's commercial hub, and its surrounding regions are characterized by towering mounds of soil and deep pits left behind by mining operations that began during an 1880s gold rush.

Boksburg, the same suburb where the recent gas leak occurred, experienced a 5.0 magnitude earthquake last month. The tremor was suspected to be linked to the extensive network of underground tunnels and shafts associated with illegal mining in the area.

The suburb previously witnessed a gas tanker explosion on Christmas Eve last year, claiming the lives of 41 individuals. A truck transporting liquefied petroleum gas (LPG) got trapped under a bridge, leading to a leak and subsequent blast.

Multiple individuals, including patients and staff members at a nearby hospital, sustained severe burns from the explosion, which also caused extensive damage to infrastructure.

zc, tg/rc (AFP, AP)