The peace mission came as NATO warplanes were in action against Kadhafi loyalists in the stricken port city of Misrata after regime forces killed at least 11 people there over the weekend, rebels said.
In Brussels, an alliance official confirmed to AFP that NATO warplanes had destroyed 11 regime tanks on the road to the eastern Libyan town of Ajdabiya and another 14 tanks near Misrata.
The African mediators were welcomed to Tripoli by Kadhafi supporters holding the veteran Libyan leader`s picture and waving the green flags of his regime.
Presidents Mohamed Ould Abdel Aziz of Mauritania, Amadou Toumani Toure of Mali, Denis Sassou Nguesso of Congo and Jacob Zuma of South Africa, had left Nouakchott in mid-morning and travelled separately to the Libyan capital.
The delegation also includes Ugandan Foreign Minister Henry Oryem Okello, representing Yoweri Museveni, the fifth head of state making up the AU panel.
Earlier, the mediators reiterated their appeal for "an immediate end to all hostilities" and proposed a transition period to adopt reforms in the insurrection-hit country.
During their two-day stay, the African leaders will meet representatives of Kadhafi in Tripoli and those of the rebels in their eastern stronghold of Benghazi, Zuma`s office said.
On the battlefield, rebels said on Sunday they had captured 15 Algerian mercenaries and killed another three during fierce fighting in Ajdabiya the previous day.
Medics also said at least 12 rebels were killed in and around Ajdabiya over the weekend.
Officials at Benghazi`s Jala hospital said it had received nine "martyrs" from the fighting and 14 wounded people, and a doctor at Al-Hawwara hospital said it had received three dead and three wounded.
Air strikes on Misrata came on Sunday morning, a rebel spokesman in the besieged town told AFP, calling the raids "a marked improvement in NATO intervention."
"They began the raids yesterday on the Kadhafi forces in the northwest of the town and near the centre of Misrata," said the spokesman.
"In the morning, there were new raids but we are not able to verify the targets," he said, adding that eight rebels were killed by pro-Kadhafi forces and 22 others were wounded on Saturday.
A doctor at Misrata hospital gave AFP the same death toll for Saturday, saying they included civilians, while putting the number of wounded at 25.
The doctor said the hospital had received three bodies on Sunday, two rebels and a civilian, adding it was possible there could be more fatalities at other medical facilities.
"As of noon today, NATO air strikes have taken out 11 tanks on the road to Ajdabiya and strikes will continue through the day and tonight," a NATO official in Brussels told AFP on condition of anonymity.
"Clearly the situation in Ajdabiya is desperate and Kadhafi forces are attacking the town with heavy weapons."
Loud explosions rocked Ajdabiya for a second day on Sunday, as rebels advanced cautiously after suffering a major reverse at the hands of loyalists.
The alliance had already taken out 15 tanks near Misrata on Friday and Saturday, bringing to 29 the total number of tanks destroyed around Libya`s third largest city in the past three days.
Western strikes against regime forces began on March 19 under a UN mandate to protect the population after Kadhafi unleashed his security forces to quell pro-democracy protests.
The United States handed control of the operation to NATO on March 31.
Libyan rebels have criticised NATO in recent days, accusing the alliance of failing to protect the people of Misrata.
But NATO says it is picking up the pace of air strikes.
Rebel spokesman Shamsiddin Abdulmolah told AFP in Benghazi that 18 men they captured near Ajdabiya were not carrying identification, but "they said they were Algerian and they had Algerian accents", adding that they would be shown in public later in the day.
"They were claiming to be selling hashish... and they had hashish with them. This is the whole crazy thing about it," he said.
He also said several Algerian ID cards and passports were found in a nearby building in Ajdabiya.
Abdulmolah said the group of 18 were led into the frontline town by a local resident allied with Kadhafi`s regime, who was also captured, adding that all the detainees were being treated well.
He accused Algeria of backing Kadhafi and of "turning a blind eye" to the mercenaries flowing into Libya.
The rebels have long accused Kadhafi of deploying hired guns from Chad and Niger against them. (*)
Editor: Kunto Wibisono
Copyright © ANTARA 2011
