Mbeki, mediating in talks to end the political and economic crisis paralysing Zimbabwe, said negotiations had not broken down and Movement for Democratic Change (MDC) leader Tsvangirai was still considering his position.
Mbeki was confident a solution to the crisis was possible. "I have no doubt that’s what will happen," he said. "If it means staying in this country for 6 months, I will do that".
A senior official of Mugabe’s ZANU-PF told Reuters a deal had already been signed with Arthur Mutambara’s MDC faction, however, a spokesman for that group said it was untrue. Mbeki said he did not know whether the deal had actually been signed.
An agreement that sidelined Tsvangirai could make it even harder to ease the crisis, which is a threat to regional stability, and to get Western countries to lift sanctions on Mugabe’s government.
Talks on power-sharing began last month after Mugabe’s unopposed re-election in a vote that was condemned around the world and boycotted by Tsvangirai because of attacks on his supporters.
But three days of meetings in Harare have failed to reach an overall deal.
"We, and the MDC headed by Mutambara have signed the agreement. Tsvangirai did not sign the agreement because he is basically trying to take us back, to renegotiate issues that we had already agreed on," the ZANU-PF official told Reuters.
"We are proceeding, and the president (Mugabe) is going to form a government of national unity including members of the opposition."
A spokesman for Mutambara’s faction of the MDC said it was untrue it had signed a deal with Mugabe’s party. He did not deny that an agreement had been reached.
The ZANU-PF official said Mugabe, in power since 1980, would form a national unity government and convene parliament next week.
Mutambara’s 10 seats would give the coalition the majority in parliament that ZANU-PF lost in March elections for the first time since independence, but excluding Tsvangirai would be unlikely to heal the deep rift in the southern African country.
"Morgan Tsvangirai is the main opposition leader, and any agreement that doesn’t include his party will not work for the country. It actually just complicates issues," said John Makumbe, a Mugabe critic and veteran commentator.
Tsvangirai’s absence in a new government would be likely to keep investors cautious over a country facing economic ruin, with the world’s highest inflation of 2.2 million percent and chronic food and fuel shortages.
Nor is it likely to bring an end to Western sanctions targeting Mugabe’s government. Western countries had demanded a big role for Tsvangirai on the grounds that he won the first round of elections but pulled out of the run-off.
Asked whether they would continue talks with Mugabe after his agreement with Mutambara, the MDC official said: "Yes, we remain committed to dialogue, but we want a fair power-sharing arrangement."
"We will now have to go back to the people and explain our position. They can go ahead and form their government. We made our position clear, that this issue is now in the hands of the facilitator," he added, referring to Mbeki.
Mbeki said he would travel to Angola on Wednesday to brief the chair of the political organ of the 14-nation regional grouping SADC and he is due to address its weekend summit in South Africa on the talks. Mbeki dismisses criticism that he has been too soft on Mugabe.
The ZANU-PF official said Mugabe had not closed the door on more talks with Tsvangirai but the ruling party refused to be "held hostage".
"The president has said they have not collapsed, and that is the position." (Additional reporting