SOUTH African mobile operator Telkom said on Wednesday bigger rival MTN Group had terminated talks to buy the former in what would have created the country's biggest telecoms company. Telkom said it could not offer MTN assurances to enter into exclusive talks sought by the country's second-biggest telecoms service provider. MTN said in a separate statement that the companies were unable to reach an agreement on a deal. In July, MTN had offered to buy its smaller rival in a stock or cash-and-shares deal. But a month later, data network provider rain, partly owned by South African billionaire Patrice Motsepe, proposed a merger with…