The conflict in Darfur in western Sudan began in 2003 after rebel groups began attacking government targets, saying their communities were being discriminated against in favour of Arabs.
Darfur, which means land of the Fur, has faced many years of tension over land and grazing rights between the mostly nomadic Arabs, and farmers from the Fur, Massaleet and Zagawa communities.
The BBC News website examines how this instability has spilled over into neighbouring Chad and the Central African Republic.
JANJAWEED AND CHAD REBELS
1. Chad accuses the Sudan government-backed Arab Janjaweed militia of attacking villagers in Chad. It says the militia has also attacked some of the 200,000 refugees that came to eastern Chad after fleeing violence in Darfur.
Chad also accuses Khartoum of backing the Union of Forces for Democracy and Development (UFDD), which is a coalition of small armed groups and army deserters who have launched cross border attacks from Darfur.
These attacks have raised communal tensions in eastern Chad, which has a similar ethnic make-up to Darfur.
2. Sudan accuses Chad of backing Darfur's National Redemption Front rebels as they carry out cross-border raids.
There have also been allegations that many of these rebels have become assimilated into Chad's national army - a charge Chad's government denies.
Some Darfur rebels come from the same Zagawa ethnic group as Chad's President Idriss Deby.
Chad has called for United Nations peacekeepers to patrol the border. Sudan is resisting any UN deployment.
3. The Central African Republic (CAR) says Sudan backs Union of Democratic Forces for Unity (UFDR) rebels who have seized towns in CAR.
The government says the UFDR are operating from Darfur with the support of the Sudanese authorities.
French forces have already deployed against CAR rebels in support of the government
4. Chad says it will send troops to help CAR fight rebels attacking northern CAR.
It accuses Sudan of attempting to destabilise both Chad and CAR and has suggested an anti-Sudan alliance.
Almost 50,000 refugees have arrived in Chad in recent weeks, fleeing fighting in CAR.