Zimbabwe's ruling party and opposition have failed to reach a power-sharing agreement at a summit of Southern African leaders in Johannesburg. But the secretary-general of the opposition Movement for Democratic Change said he believed a deal with the Zanu-PF would be reached "very soon". South African President Thabo Mbeki, who is mediating in the crisis, earlier said talks would continue. The MDC is insisting President Robert Mugabe surrender his executive powers. MDC leader Morgan Tsvangirai is said to have agreed in principle to take on a powerful prime ministerial role in a new national unity government, although the exact balance of power is undecided. In other business, the 14-member Southern African Development Community (Sadc) agreed to launch a regional trade zone aimed at eliminating import tariffs, with plans for a common currency by 2018. Zimbabwe is among a majority of Sadc countries who will participate in the trade zone. Angola and the Democratic Republic of Congo did not join up immediately. 'No deal yet' The BBC's Alan Little in Johannesburg says both sides in the negotiations have agreed on many things - that there should be a power-sharing government, that Mr Mugabe should be president and Mr Tsvangirai prime minister. But the opposition has insisted that the president should cede real executive power to the MDC leader and stay in office only as a ceremonial head of state. Mr Tsvangirai is prepared to share cabinet posts with members of Zanu-PF, but he wants ministers to be answerable to him rather than Mr Mugabe. This, our correspondent says, Mr Mugabe refuses to concede. Although Mr Tsvangirai had earlier said the talks were going "very well", as the summit formally ended his spokesman told reporters: "There is no deal yet." Sadc's mediator for Zimbabwe, Thabo Mbeki, said negotiations would continue, but added that it might "be necessary to convene parliament" during that time. While stressing that there was an urgent need to form an inclusive government in Zimbabwe, the South African president warned that a solution to the crisis was doomed unless all parties agreed to it. "It won't last unless it's a common product that is owned by this entire collective of the leadership of Zimbabwe," Mr Mbeki said. Sadc's committee on security and politics reviewed a report on the mediation effort and concluded the agreements so far achieved on less contentious issues were "a good basis for a global agreement". Later, MDC Secretary-General Tendai Biti told a news conference in Johannesburg that his party was committed to the negotiations and that failure was not an option. "We trust that there will be a conclusion... very soon," he said. "There are issues that are outstanding, but quite clearly one has no business in negotiating if you are not prepared to compromise. "It is critical that we conclude this dialogue as a matter of urgency." Mr Tsvangirai finished ahead of Mr Mugabe in the first round of Zimbabwe's presidential election in March and his Movement for Democratic Change also won a majority in parliamentary elections. But the opposition leader pulled out of the second round of the presidential election, citing a campaign of violence against his supporters. Mr Mugabe went on to win the vote unopposed. On Sunday, Mr Tsvangirai told the New York Times that the most basic issue of how he and Mr Mugabe would share power remained unsettled, and there were limits to the compromises he could make. "It's better not to have a deal than to have a bad deal," he said. Mr Tsvangirai had a seat at the Sadc summit with other invited guests on the floor while President Mugabe joined other regional leaders at the head table. Arthur Mutambara, the head of a breakaway MDC faction, also attended. Source: BBC