Zimbabwe President Robert Mugabe and rival Morgan Tsvangirai resumed power-sharing talks on Friday under mounting international pressure to end a feud over the most important cabinet posts.

They have already held three days of negotiations, mediated by former South African president Thabo Mbeki, but have shown little sign of nearing agreement on how to share control over the most powerful ministries.

As he arrived for the talks, Mugabe told reporters: "It's a day for deals."

But Tsvangirai sounded more cautious when asked if he believed a deal would be struck, saying: "We all have to have hope, don't we?"

An official in Tsvangirai's Movement for Democratic Change (MDC) warned that the talks could collapse if Mugabe refuses to cede control of key ministries.

"If they do not shift their position, there is no deal," the official said, speaking on condition of anonymity.

State media accused the MDC of negotiating in bad faith to provoke deadlock and then calling for intervention by the United Nations.

"They want Mbeki to fail"

"They want Mr Mbeki to fail so that there is an excuse to involve the UN and thereby internationalise the matter and make it easier to pursue their own agenda in Zimbabwe," the Herald daily said.

MDC spokesperson Nelson Chamisa insisted that the party would not back down on its demand for crucial ministries, saying some advances had been made "but not enough to seal the deal."

Under the power-sharing agreement signed one month ago, 84-year-old Mugabe would remain as president while Tsvangirai would take the new post of prime minister.

But Tsvangirai has threatened to pull out of the deal, after Mugabe awarded key cabinet posts to his own party, leaving him with a firm grip on Zimbabwe's security forces.

"We are very clear we are not going to compromise," Chamisa said. "There is no going back on fundamental issues, we will not play second fiddle to Zanu-PF."

Mbeki flew to Harare in hopes of saving the deal that he brokered to rescue Zimbabwe from economic and political chaos.

UN threatening to impose more sanctions

But Western powers have taken a dim view of his chances for success. The United States on Friday joined the European Union in threatening to impose more sanctions against the regime if the unity accord falls apart.

"If it doesn't work then we are going to continue the pressure that we've put on the government. We will look at new sanctions against President Mugabe and his regime," Jendayi Frazer, the top US diplomat for Africa, told reporters during a visit to Tokyo.

"Right now we're not so optimistic. It doesn't look very good for power-sharing," she said.

Western countries have insisted that any deal must respect the outcome of the first round of presidential balloting in March, when Tsvangirai handed Mugabe his first electoral defeat since independence from Britain in 1980.

But the former union leader failed to win enough ballots to declare an outright victory in the presidential poll. He pulled out of the run-off in June, accusing the regime of coordinating a brutal campaign of political violence that left more than 100 of his supporters dead.

As the political crisis drags, Zimbabweans face a daily struggle for survival in a country buckling under the world's highest inflation rate, at 231 million percent.

Once one of Africa's most prosperous nations, Zimbabwe's stunning economic collapse has caused critical food shortages, with nearly half its people needing UN aid and 80 percent of the population living in poverty.