(Adds Chamber of Mines comment)
JOHANNESBURG, Oct 31 (Reuters) - South Africa's National Union of Mineworkers (NUM) said on Tuesday its coal workers would call a strike next week if wage demands were not met by the Chamber of Mines, the body that represents several mining firms in pay talks.
A strike in the coal sector could affect the nation's power supply, as most of it is generated from the fuel.
Labour unrest in the mining industry has cost South Africa billions of dollars in lost output in the past.
The union said it had given Chamber of Mines until Nov. 8 to increase the current wage offer.
If it failed to act, NUM spokesman Livhuwani Mammburu said: "We will declare a strike and that is going to have devastating economic consequences."
The chamber, which represents firms such as Anglo American Coal, Exxaro and Glencore, said the offers made were fair.
"The chamber believes that the offers made are fair and reasonable. Employees in the coal sector are amongst the highest paid employees in the mining industry," said the chamber's head of employment relations and lead negotiator for the coal industry, Motsamai Motlhamme.
The wage offers vary depending on the company. They range from a 5 percent to 7.5 percent increase in 2018 and in 2019 for the lowest paid workers. Some of the mining companies are also proposing once-off payments as part of the wage offers.
The union is demanding a once-off payment of 1,100 rand ($78) this year and an 8 percent increase for 2018.
The NUM's demands also include a 9 percent wage rise in 2019. The union said it would only sign a deal for a third year if the chamber agreed to stay in the collective bargaining forum.
The chamber and unions agreed in June to negotiate wages collectively for 2017, defusing a potential source of friction after NUM threatened to go on strike.
Much of the industry is still controlled by South Africa's white-minority more than two decades after demise of apartheid, while black people make up most of the labour force.
In a separate development, NUM said it had signed a two-year wage agreement with Modikwa Platinum Mine on Monday, a subsidiary of African Rainbow Minerals.
$1 = 14.0775 rand Reporting by Tanisha Heiberg; Editing by James Macharia and Edmund Blair