By Devon Maylie and Clare Connaghan

JOHANNESBURG--Industrial action that has severely disrupted South Africa's mining industry spread briefly to the world's largest platinum producer Friday morning, after 100 employees at Anglo American Platinum Ltd.'s (AMS.JO) Thembelani mine refused to go underground.

Anglo American Platinum, also know as Amplats, said it quickly resolved the dispute and by mid-morning all employees had reported to work. But political tensions associated with the long-running labor dispute continued to rise as South Africa's trade union federation, Cosatu, singled out politicians it said were opportunistically exploiting workers' problems that lie at the heart of the violent unrest this year at some of the country's mines.

The continuing mining unrest, and its broader political implications, prompted ratings agency Fitch to warn of broader structural problems in South Africa's society and economy that could weigh down its credit rating in the future.

"The protests alone do not directly affect the sovereign rating, but they do highlight structural problems that may weigh on it," the ratings company said. "In the longer term...failure to speed up growth and sustain job creation will weaken South Africa's credit fundamentals."

Strikes by workers demanding better pay and conditions at some of South Africa's largest platinum mines have disrupted production of the metal and provoked some of the worst violence seen in the post-apartheid era.

During an illegal strike at Lonmin PLC's (LMI.LN) Marikana mine last week, police shot and killed 34 workers in a crowd where many were armed with machetes and spears. Earlier that week, fighting among workers left 10 people dead, including two police officers.

Cosatu said Friday that the blame for the violence lay with "bogus breakaway 'unions' and their political and financial backers," who are using intimidation and violence to try to divide workers.

Rivalry between the well-established Cosatu-affiliated union National Union of Mineworkers and the emerging Association of Mineworkers and Construction Union also sparked violent illegal strikes at Impala Platinum Holdings Ltd. (IMPUY) in February.

The disturbances, in particular the deaths, have pushed the government and the companies to act as accusations fly over who and exactly what was responsible for the unrest. There have also been copycat wage demands at other mines in the region this week.

Amplats experienced its first disruption Friday, although the company said it quickly held talks, which "concluded on a positive note" with representatives of the 100 workers who refused to go underground. "The [workers'] representatives have committed to engage with management through the mine-based channels going forward," it said in a statement.

Cosatu singled out Julius Malema, the expelled leader of the ruling ANC's youth league, as an opportunist exploiting the situation for political gain.

"Julius must stop. It's not helping anybody, fomenting divisions in society because he's been expelled from the ANC," said Cosatu President Sdumo Dlamini.

Malema, an opponent of South African President Jacob Zuma, warned in a speech at a memorial for the dead Lonmin workers Thursday that the country's mines should brace for a revolution unless conditions improve.

A ministerial committee accused Mr. Malema of turning the memorial into a political rally when he addressed the crowd.

The current situation in South Africa "over time could also threaten social and political stability, damaging the investment climate further," warned Fitch.

Write to Devon Maylie at devon.maylie@dowjones.com

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