The formation of the new government, which became possible after a January peace deal, was delayed after John Garang, the former leader of the Sudan People's Liberation Movement (SPLM), died in a helicopter crash on 30 July.
Salva Kiir, who took over as leader of the SPLM and first vice-president of Sudan, was to meet President Omar Hassan al-Bashir late on Saturday for talks that may last until Monday, an official from the former rebel group who declined to be named said.
"This is a really important meeting ... The two sides have decided most of the details. There are a few major things left to discuss and then the new government will be formed," the official said. "According to our schedule we should have a new government by 7 September."
SPLM officials said earlier they expected the announcement of a new government by the end of August or early September.
Meanwhile other Sudanese parties have rejected the distribution of ministerial posts in the new provisional government.
According to the peace deal that ended a more than 20-year rebellion, the ruling National Congress Party will form 52% of the government, the SPLM will form 28% and other political groups will form the remainder.
"This is a really important meeting ... The two sides have decided most of the details. There are a few major things left to discuss and then the new government will be formed"
The National Congress Party held a meeting on Saturday to finalise the names of officials it would hand positions to.
The SPLM official said the group had also decided on officials it would place in the positions under its control.
But he added, among the issues still to be decided was the control of the budding oil ministry.
Political commentator Faisal Muhammad Salih said after the formation of the new government the SPLM will concentrate on building bridges with other southern groups, some of whom allied with the government against the SPLM during the civil war.
"There will be more emphasis on south-south dialogue after the new government is decided," he said, adding that Kiir had created a good atmosphere for compromise among southern leaders.
Analysts have said Kiir has a more collegial style of leadership than Garang, who maintained a tight grip on power.
SPLM officials say progress has been made on incorporating a number of southern militias into the group's forces.