Thirteen South African mine workers have been injured during fierce clashes between two rival unions in Rustenburg, police say.A large mob attacked union stewards with machetes and sharp objects at the Anglo American Platinum mine.Security guards fired rubber bullets at the mob, injuring nine miners. Four guards were also wounded in the fight.Last August, Rustenburg was the scene of violent wildcat strikes, during which police shot dead 34 miners.The bloodshed at the Marikana mine was the most deadly police action since South Africa became a democracy in 1994 and sparked national outrage.No officers were involved in Monday's incident, police spokesman Thulani Ngubane confirmed to the BBC.The stand-off is thought to have taken place between the National Union of Mineworkers (NUM) - allied to the ruling African National Congress (ANC) party - and the militant Association of Mineworkers and Construction Union (Amcu).They may have been fighting over the occupancy of union offices at the Anglo American Platinum's Siphumelele mine shaft in Rustenburg, 100km (60 miles) north-west of Johannesburg, Mr Ngubane said."Mine security intervened and fired rubber bullets, and in the process a total of 13 mine workers, including four security guards [were injured]," Mr Ngubane said.Some of the people injured were hacked with machetes and sharp objects, he added.Three of the wounded miners are believed to be in a critical condition.Last year, wildcat strikes and violent unrest over wages spread to a number of mines across South Africa, the world's largest platinum producer.