JOHANNESBURG (Reuters) - Glencore Xstrata Plc sacked 1,000 workers across three of its chrome mines in South Africa for going on illegal strike last week, bringing those operations to a standstill, the company said on Monday.

The dispute at the mines near Steelpoort, northeast of Johannesburg in Limpopo province, added to long-running friction in the mining industry that has caused production to slow, raised concerns about Africa's largest economy and sent the rand to fresh four-year lows.

Chromium is a raw material used to produce ferrochrome, a key ingredient to make stainless steel.

"About 1,000 of the employees who have participated in the unprotected (illegal) strike have been dismissed," said Christopher Tsatsawane, a spokesman for the company's chrome operations.

The strike, which started last Tuesday, was continuing, but supplies to customers were not yet affected, he added.

The workers have until Tuesday to appeal the dismissals.

South Africa has well-defined processes for launching strikes and those who fail to get formal approval can be sacked.

But employers often hire back most or all the workers they fire. The dismissals are seen as a hard-ball tactic to force labor into making a deal.

The miners, most of whom belong to the militant Association of Mineworkers and Construction Union (AMCU), stopped work in solidarity with an individual who says he was assaulted by a shift supervisor, the company said last week.

Talks over the weekend between AMCU and the company failed to end the dispute and no other meetings were scheduled at this stage, Tsatsawane added.

Dismissal of workers is likely to increase tension with trade unions as they head into wage negations in the coming weeks.

South Africa, home to around 75 percent of the world's chromite reserves, has become a flashpoint of violent labor strife as AMCU and the dominant National Union of Mineworkers (NUM) as they battle for members.

(Reporting by Agnieszka Flak; editing by Jon Herskovitz and Keiron Henderson)

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