* AU chairman and UN envoy visit Kidal
* Met rebel groups to negotiate ceasefire and talks
* Residents flee south in fear of renewed fighting (Recast with AU-UN visit to Kidal)
By Tiemoko Diallo
BAMAKO, May 23 (Reuters) - The chairman of the African Union met armed Tuareg separatist groups in northern Mali on Friday to discuss a permanent ceasefire and resumption of talks between the rebels and Mali's government, officials said.
The discussions between Mauritanian President Mohamed Ould Abdel Aziz and the Tuaregs in the town of Kidal came after fighting this week that threatens to sink struggling peace negotiations and plunge Mali's desert north back into war.
The United Nations special representative in Mali Albert Koenders accompanied Abdel Aziz in the meetings. He said in a statement progress was being made in establishing a ceasefire, the release of prisoners held by rebel groups and resumption of political dialogue between the two sides.
"The objective of the meeting is to obtain an immediate ceasefire and get the parties to the negotiation table," Mauritanian state news agency AMI said, quoting Mauritanian officials.
A source in the Tuareg delegation at the meeting told Reuters the rebels wanted the Malian government to accept responsibility for the clashes.
Mali's defence minister Soumeylou Boubeye Maiga said late on Thursday about 20 soldiers had been killed and 30 others wounded in a failed attempt by government forces to retake Kidal, the Tuareg separatist stronghold.
The army launched Wednesday's assault on Kidal after clashes erupted at the weekend during a visit to the town by new Prime Minister Moussa Mara.
Eight civilians, including six government workers, were killed when the separatists attacked the local governor's office. Another 32 civil servants taken hostage by the rebels were later released.
The government has accused the separatists, including the National Movement for the Liberation of Azawad (MNLA), of renewing their former alliances with al Qaeda-linked Islamist groups.
"The MNLA was supported by its traditional allies," Maiga said.
Mali was plunged into chaos in 2012 after Tuareg independence fighters teamed up with armed Islamist groups to seize the north following a coup in the capital.
When they were sidelined by the better-equipped Islamists, the separatists broke with their allies. A French-led military operation drove the Islamists back last year.
The MNLA says it controls at least seven northern towns in addition to Kidal and that government troops either abandoned their positions and sought refuge at the camps of the U.N. peacekeeping mission, MINUSMA, or fled south on Wednesday.
While neither the U.N. nor French forces in Mali intervened to halt this week's clashes, MINUSMA said it had protected 62 Malian soldiers in its base in Kidal and another 290 troops in the town of Aguelhok.
The mission said it had airlifted a total of 61 wounded Malian soldiers to the northern city of Gao and the capital Bamako on Thursday.
Fear of renewed fighting has pushed some people in the northern towns of Kidal, Gao and Timbuktu to once more seek refuge in the south, residents said.
"Many people have fled the city, especially state officials. All local administrators and regional directors of public services, including bank managers have all fled. Banks are closed in Timbuktu," said Ada Cissé a local councillor in Salam near Timbuktu. (Additional reporting by Adama Diarra in Bamako and Laurent Prieur in Nouakchott; Writing by Joe Bavier and Bate Felix; Editing by Andrew Roche)