UN leader Ban Ki-moon on Wednesday called on Tuareg rebels in Mali to halt their attacks and begin talks with the government amid mounting international worries over the unrest.More than 20,000 people have fled from northern Mali into neighboring Burkina Faso, Algeria and Mauritania, according to the UN. Ban is \"deeply concerned\" over civilian casualties and the humanitarian impact of the conflict, said his spokesman Martin Nesirky.The battle between Tuareg rebels and government forces is \"aggravating an already dire humanitarian situation throughout the Sahel region,\" said the UN spokesman.Ban called on the rebel groups \"to immediately cease their attacks and to engage in dialogue with the Government of Mali to resolve their grievances.\" He offered UN support for efforts to find a peaceful solution.The Azawad National Liberation Movement (MNLA) and other Tuareg rebels -- boosted by the return of men who fought in Libya for toppled leader Moamer Kadhafi -- are demanding greater autonomy for their nomadic desert tribe.