Whole bunch of infighting occurring in Northern Mali
http://en.wikipedia.org/wiki/Tuareg_rebellion_(2012)#Capture_of_Timbuktu_and_Douentza
The big force is the MNLA, the "National Movement for the Liberation of Azawad" is the main opposition in most of Northern Mali. The Islamists, Ansar Dine, only joined in the revolt around March 2012 and then as a Sunni radical group that had access to weapons, it is believe to be from Libya.
http://en.wikipedia.org/wiki/National_Movement_for_the_Liberation_of_Azawad
http://en.wikipedia.org/wiki/Ansar_Dine
Ansar Dine took Timbuktu from the MNLA in November 2012, thus the recent attack was against Ansar Dine not the MNLA.
Part of the confusion is both the MNLA and Ansar Dine seems to be Tuareg dominated and given that the Tuareg are spread through four countries of African (and have greater loyalty to their tribe then what ever "Country" they are in), they can do do expect help from other tribesmen from other countries.
http://en.wikipedia.org/wiki/Tuareg
Thus the MNLA seems to be dominated by Tuareg who live in Northern Mali, while the Ansar Dine have Tuareq members from all of Tuareq dominated areas. The MNLA seems to be willing to cut a deal with the French, while the Ansar Dine do not want to UNLESS it includes all of Tuareq lands in all four countries, Libya, Algeria, Niger Morocco and Burkina Faso. To the French what the MNLA wants is something they can live with, what Ansar Dine wants is unacceptable for it would divide up what was known as "French North Africa" prior to the 1950s.
Surprising the Tuareq are a matrilineal Tribe (i.e. trace they ancestry through their mothers), even through they embraced Islam after the Arab Conquest of the 700s.
The problem of African Borders (Otto von Bismark plan to divide up Africa so it would cause wars is still working):
http://mycontinent.co/AfricaBorders.php
Please note the Tuareq is a tribe within the larger Berber group:
http://en.wikipedia.org/wiki/Berber_people

Non-Tuareq Berbers tend to have more intermixing with other groups (mostly Arab) but many still retain loyally to the Concept of being a Berber over what ever citizenship they have (and why the Call of Religion is more important then that of Nationality, it permits unity with people other then their relatives). Thus it appears many of the "Islamists" are Berbers of tribes other then the Tuareq and are fighting for a greater "Berber" unity tied in with Islam (Thus including the Local Arab population also). Thus the make up to the Ansar Dine is heavily united based on religion while the MNLA is more a product of tribal unity.
Another way to look at this is the MNLA is willing to cut a deal if they are given more rights within Mali, but the Answer Dine wants more power through out North Africa,not just in Tuareq land.