The agreement is seen as a chance to prevent an economic collapse that could put added strain on neighbours which already host millions of Zimbabweans who fled in search of work and, more recently, to escape a deadly cholera epidemic.
An official of opposition leader Morgan Tsvangirai’s Movement for Democratic Change (MDC) said the 15-member regional SADC bloc summit had not persuaded the rivals to implement the power-sharing deal signed last September.
"We are worlds apart. If we were (inches) apart we are now miles apart," the MDC official told Reuters.
Mugabe and Tsvangirai signed the agreement in September but have failed to agree on control of cabinet posts, with neither side showing any sign of compromise.
"Questions concerning Zimbabwe are continuously being raised in capitals and streets of Africa, with the expectation that the Zimbabwean leadership of all persuasions, under the aegis of SADC, will resolutely resolve the impasse with decisiveness and statesmanship," South African President Kgalema Motlanthe told the summit. "I trust that we will not fail them."
Mugabe, in power since 1980, and his ZANU-PF party have urged the opposition to join a unity government but say they will not hesitate to form one without them.
Mugabe is expected to seek approval from regional leaders at the summit in Pretoria to form a government alone if need be.
Western leaders want Mugabe to step down and are pushing for a democratic government to embrace economic reforms before billions of dollars in aid is offered, but he has resisted their calls through several rounds of negotiations.
In Brussels, the European Union stepped up pressure on him on Monday by adding 27 individuals and 36 firms to a sanctions list and calling for a probe into Harare’s diamond industry, EU officials said.
A Zimbabwean deputy minister billed Monday’s summit as the last chance for rescuing the power-sharing pact, viewed as the best hope for Zimbabwe, where prices double every day and cholera has killed nearly 2,900 people since August.
"The way forward soon after this summit, whether there is an agreement or there is no agreement, President Mugabe is going to form a cabinet," deputy Information Minister Bright Matonga told South African public broadcaster SAFM radio.
He said Mugabe would try to leave room for Tsvangirai if he decided to change his mind, but not for long.
Tsvangirai says ZANU-PF is trying to sideline him and wants control of powerful ministries such as Home Affairs. He says no deal is possible unless party activists are released from jail.
Within the SADC bloc, Zambia and Botswana have taken a tough line on Mugabe, but other members favour a more diplomatic approach with the man they still revere as a liberation hero.
Botswana’s President Seretse Khama Ian Khama, one of Mugabe’s toughest critics, attended the summit after boycotting one in August.
Without a political settlement, it is unlikely sanctions imposed on Zimbabwe’s leadership by Western countries will be lifted