South Africa's new President Kgalema Motlanthe has said his predecessor, Thabo Mbeki, should continue to lead mediation efforts in Zimbabwe. Mr Mbeki brokered a power-sharing deal between President Robert Mugabe and his long-time rival, Morgan Tsvangirai, before stepping down last week. Mr Tsvangirai's party says talks on forming a cabinet are deadlocked and outside mediation is again needed. But the president's Zanu-PF party has denied there is a stalemate. Mr Motlanthe's statement comes as the UN said half of Zimbabwe's population could soon be in need of constant food aid and medical assistance. In the deal signed two weeks ago, the opposition Movement for Democratic Change (MDC) and Mr Mugabe's Zanu-PF agreed to work together with Mr Mugabe as president and Mr Tsvangirai as prime minister. Mr Motlanthe said he had confidence in Mr Mbeki's "ability to build on the historic successes already made in the power-sharing negotiations under his mediation". Mr Mbeki resigned amid claims - which he denies - of political interference in a corruption case against South Africa's governing party leader, Jacob Zuma. 'Mischievous' According to MDC spokesman Nelson Chamisa, Zanu-PF are demanding all the key ministries in the new unity government. "The arrogance being shown by Zanu-PF can only be dealt with through mediation. They need someone to help them see the bigger picture," Reuters news agency quotes him as saying. But chief Zanu-PF negotiator, Patrick Chinamasa, said talk of a deadlock was "mischievous". "If there was a disagreement as is being suggested, I don't think it's one that would justify calling in the facilitator," Mr Chinamasa said, AFP news agency reports. Mr Mbeki was appointed by the Southern African Development Community to resolve Zimbabwe's problems. It was hoped that a new government would overcome the acute economic crisis as the official inflation rate is still about 11m% and there are severe food shortages. UN humanitarian chief John Holmes told the BBC that three million people were already reliant on aid, and that figure could rise to five million. He acknowledged that access for aid agencies had improved greatly since the deal, but preparations needed to be made for next year's harvest to avoid millions more people becoming reliant on aid. Ahead of the power-sharing agreement, the government lifted a ban on the distribution of food aid that had stopped aid agencies going to rural areas. Critics had accused the ruling party of not distributing aid to opposition areas. Source: BBC