"We agreed at the organ [security troika] that the agreement should be signed within the period of the summit," said a foreign minister at the meeting who declined to be named.
Referring to the Zimbabwean officials, the minister said: "All the parties appear to be agreeable [to reaching an agreement this weekend], but it's a wait and see situation."
Tendai Biti, chief negotiator for the MDC said his party has resumed talks with Mugabe's Zanu-PF.
Biti sat with Tsvangirai just behind cabinet ministers from the region at the summit, while Mugabe sat at the front table.
Asked if negotiations were back on track after Tsvangirai walked out of a session in the Zimbabwean capital earlier this week, Biti said the parties are "talking here".
Zimbabwe's political crisis intensified after Mugabe's re-election in a June presidential run-off was widely condemned as a sham.
Tsvangirai boycotted the poll despite finishing ahead of Mugabe in the first round of the election, citing rising violence against his supporters.
Sticking points remain
A South African official close to the talks told AFP news agency that the remaining sticking points in the power-sharing negotiations include whether Mugabe will retain the right to hire and fire ministers.
A timeframe for how long a government of national unity would remain in place is also being debated, with the MDC pushing for a shorter arrangement and the ruling Zanu-PF party wanting a longer length of time.
The opposition also wants a clause included to specify that new elections would be held within 90 days if any of the parties were to withdraw from the government.