Harare - Zimbabwean opposition leader Morgan Tsvangirai has flown to South Africa for medical care, a party official told AFP on Saturday, as concerns grow over his health ahead of next year's election.Tsvangirai, a former prime minister and veteran opponent of President Robert Mugabe, announced last year that he had been diagnosed with colon cancer and had begun chemotherapy.Tsvangirai hopes to lead a united opposition into 2018's general elections as Mugabe seeks to extend his 37-year-long stranglehold on power."President Tsvangirai went to South Africa for medical reasons," a Movement for Democratic Change (MDC) party official told AFP. "He is recovering."Tsvangirai, 65, was also flown to South Africa in September for medical treatment before being discharged and returning to Harare in early October.He won the most votes in the first-round of the 2008 presidential elections, but poll officials said it was not enough to avoid a run-off against Mugabe.As Zanu-PF ruling party loyalists unleashed a wave of violence, Tsvangirai pulled out of the race and became prime minister in a power-sharing government in which he was widely seen as being outmanoeuvered by Mugabe.Mugabe, 93, regularly flies to Singapore, reportedly for health reasons. Details of his medical condition are kept under wraps, but he recently said doctors had given him a clean bill of health. The World Health Organisation briefly appointed Mugabe as a goodwill ambassador in before rescinding the offer last week following widespread outrage.