Fierce fighting is continuing in the eastern town of AjdabiyaA team of African leaders is on its way to Libya to try to negotiate a ceasefire between rebel forces and those loyal to Col Muammar Gaddafi.
South African President Jacob Zuma and three other leaders, representing the African Union, have left Mauritania for the Libyan capital Tripoli.
The team will also visit rebel representatives in Benghazi.
Fierce fighting is continuing in Ajdabiya in eastern Libya, with Col Gaddafi's forces pushing back rebels.
'Dialogue'The African Union (AU) diplomatic mission comprises representatives from five nations and had gathered in Mauritania's capital, Nouakchott.
The mission has called for an "immediate end" to fighting, "diligent conveying of humanitarian aid" and "dialogue between the Libyan parties".
Agence France-Presse news agency said Mr Zuma, and presidents Mohamed Ould Abdel Aziz of Mauritania, Amadou Toumani Toure of Mali and Denis Sassou Nguesso of the Republic of Congo, along with Uganda's Foreign Minister Henry Oryem Okello, were travelling on separate planes.
South African President Jacob Zuma is part of the AU missionEarlier, a statement from the South African presidency said: "The [African Union] committee has been granted permission by Nato to enter Libya and to meet in Tripoli with.. [Col] Gaddafi. The AU delegation will also meet with the Interim Transitional National Council in Benghazi on 10 and 11 April."
The five-strong panel was approved by the European Union to mediate in Libya.
Panel spokesman Abdel Aziz said: "The main objective of the panel is to put an end to the war and to find an adequate solution to the crisis."
The mission will face a difficult task as the rebels have refused to discuss a ceasefire without the removal of Col Gaddafi and his family.
The BBC's Jon Leyne in Benghazi also says that the AU team's plan for the two sides to work together in a transition to democracy looks to be a non-starter.
He says it appears that neither side appears to be ready to make the compromises necessary for a ceasefire.
'Helicopters downed'Fierce fighting is raging for a second day in Ajdabiya.
Heavy gunfire and loud explosions were heard in the town on Sunday, with reports of intense shelling of the town from the west, from where pro-Gaddafi forces are attacking.
Ajdabiya is the last town before the rebels' main city of BenghaziOne rebel to the east of Ajdabiya told Reuters: "There is resistance inside the city. Gaddafi forces are fighting with rebels. They have a presence inside."
Another said: "There are Gaddafi forces inside Ajdabiya in sand-coloured Land Cruisers and we know there are Gaddafi snipers in civilian clothing in the city as well."
Ajdabiya is important to the opposition as it controls a strategic crossroads and is the last town before the main rebel city of Benghazi.
Rebel forces had advanced towards Brega on Saturday but were forced back by a counter-attack.
Our correspondent, Jon Leyne, says that once again Libyan government forces have shown they are able to operate in a much more sophisticated way than the opposition.
They outmanoeuvred the rebels by coming in from the desert.
Our correspondent says the rebels claimed to have captured Algerian mercenaries from Col Gaddafi's forces, though this cannot be independently verified.
Meanwhile, Libyan Deputy Foreign Minister Khaled Kaim said government forces had shot down two rebel helicopters in the east, but this also cannot been confirmed.
He said: "A clear violation was committed by the rebels to [UN] resolution 1973 relating to the no-fly zone."
Nato says it is applying the zone fairly and on Saturday escorted a rebel MiG-23 fighter jet back to its base.
Nato is continuing its air strikes on Gaddafi military targets as it pursues the UN resolution to protect civilians.
It said on Saturday it had destroyed another 17 tanks and damaged nine others, many around the western city of Misrata.
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