Shanduka Coal said about 250 employees at the Graspan colliery east of Johannesburg submitted a list of demands on Tuesday before seizing mine equipment and refusing to disperse.

"The company remains committed to negotiations on issues of concern within the appropriate legal framework to resolve the matter amicably," Shanduka Coal said in a statement.

Police fired rubber bullets at the demonstrators when they tried to charge their lines with earth-moving equipment, police and the company said Wednesday.

Operations at Graspan remained suspended following the wildcat protest, which left seven injured and another nine under arrest.

Shanduka said the strike was unlawful and a breach of worker contracts.

"The police dispersed the striking employees, who were on mine premises illegally, had seized mine equipment and were refusing to leave peacefully," it said in a statement.

Shanduka Coal has a production capacity of about 9 million ROM MT and employs 1500 people, according to Glencore’s website.

The unrest in the coal industry began at two Exxaro mines over a fortnight ago and has now escalated to involve six of the company’s mines and about 3200 of its employees.

The strikes at Exxaro also turned violent this week, with police firing rubber bullets at strikers at one mine.

At the latest meeting between the two parties, Exxaro’s eight-point proposal, which included a one-off offer of R2000 ($US216) to end the strikes, was rejected.

Often-violent labour unrest shut parts of South Africa's mining industry for months last year and the industry remains anxious about a renewal of the disturbances that left more than 50 people dead and pushed prices higher.

The stoppage of production at the affected mines also puts pressure on state utility Eskom, which uses coal to generate 85% of South African electricity.

Exxaro is the second-largest supplier of coal to Eskom and there are fears the strikes could eventually lead to an electricity shortage and blackouts.