(CNN) -- Libya's new leadership started a three-day meeting Sunday to discuss the formation of an interim government, a senior council member said.
The formation of a government could take up to a week, but National Transitional Council members have agreed it would include a premier, a vice premier and 22 ministers, council member Mohammed Naser told CNN.
Leaders are meeting in Benghazi a day after their fighters claimed a new military advance into Sirte, the hometown of deposed Libyan dictator Moammar Gadhafi.
Twenty-four hours of NATO aerial bombardments preceded the advance.
The Western military alliance "struck a series of targets (Gadhafi) forces used in order to threaten the civilian population of Sirte," NATO said in a statement.
"Among the reports emerging from Sirte are executions, hostage-taking, and the calculated targeting of individuals, families, and communities within the city," according to NATO, which also pointed to mercenaries being involved on the pro-Gadhafi side and civilians denied access to food, water and medical care.
The NTC said Sunday its fighters had repelled an attack by Gadhafi loyalists on the town of Ghadames. In the attack, which occurred near dawn Saturday and lasted until early Sunday, the Gadhafi loyalists used rocket-propelled grenades, anti-aircraft guns and machine guns. Col. Abdulbast Al-Wahidi, a military commander, said four rebel fighters were killed and several injured in the incident.
"They also sustained losses we could not confirm, but we know they are retrieving now the dead bodies of their men," Al-Wahidi said.
The NTC believes the loyalists are related to Khamis, a son of Gadhafi's, Col. Ahmed Bani, military spokesman for the ministry of defense, told reporters Sunday.
Also Sunday, an NTC committee announced it had located a suspected mass grave with 1,270 bodies, believed to be victims of a 1996 massacre at Tripoli's Abu Salim prison. It was unclear, however, whether the site actually was a mass grave, as no excavations have taken place.
Last week, a senior member of the transitional council told CNN that the formation of a government would not be announced until cities under control of Gadhafi loyalists are liberated.
The council said it will expand as cities are liberated to ensure representation in all regions of the country.
On Sunday, Zimbabwe leader Robert Mugabe -- who has himself previously drawn international criticism -- said African leaders would only recognize the NTC as Libya's legitimate leaders if talks take place between the NTC and Gadhafi loyalists.
"As far as we are concerned, the African position is that the NTC can only have a seat in the AU if the summit of the AU recognizes that in fact they are in control, they are the de facto authority of Libya," Mugabe, 87, said upon his arrival back in the country from the United Nations. "We will not go as far as European and NATO countries to recognize (the NTC) as the absolute authority as we still want negotiations between the NTC and the Gadhafi loyalists."
Last week, South Africa, Algeria and the African Union recognized the NTC as Libya's legitimate rulers.
"The period in which we are is a period in which there appears to be a reversal of what the founding fathers did," Mugabe said. "Principles are being sacrificed on the altar of expediency because you are being assisted by the West, therefore you must bow to the West. It is a terrible, terrible period. It is a selling out of the principles of the (African) founding fathers."
CNN's Mohammed Tawfeeq and journalists Mohamed Fadel Fahmy and Columbus Mavhunga contributed to this report.