The violence in Chad could turn into a genocide similar to that in Rwanda in 1994, the UN refugee agency has warned.
Killing tactics used in Darfur are being used in Chad
The UNHCR says the killing tactics from neighbouring Darfur in Sudan have been transported to eastern Chad in full.
The warning comes as Chad, Sudan and the Central African Republic signed a deal not to support rebels attacking each other's neighbouring territory.
African Union head, Ghana's President John Kufuor, said they seemed ready to agree to an AU/UN border peace force.
"They seem to be ready to accept a beefed-up force from the African Union and the United Nations to take control of the borders among them," Mr Kufuor told reporters at the French-African summit in Cannes where the declaration was signed.
More than 200,000 people have been killed and 2.5m displaced since war broke out in Darfur four years ago.
Concern is now growing for the 200,000 refugees who sought shelter in eastern Chad.
The conflict in Darfur has followed them across the border with attacks by Janjaweed Arab militia on camels and horseback leaving hundreds dead and 110,000 people homeless.
The BBC's Orla Guerin in eastern Chad says at first, the Janjaweed came from Sudan; later, locals joined in - neighbour killing neighbour.
"We are seeing elements that closely resemble what we saw in Rwanda in the genocide in 1994 and I think we have an opportunity here to avoid such a tragedy from occurring again," UNHCR's Matthew Conway said.
Meanwhile, in Sudan, UN special envoy to Darfur Jan Eliasson is trying to arrange face-to-face talks between the Sudanese government and the rebels in Darfur.
He said the main concerns of the rebel groups that had not signed last May's peace deal were compensation power-sharing and security.
"With readiness on the government side to open up for amendments and improvement then I think there is room for negotiation," he told the BBC's Network Africa programme.
"We'll try to bring the horses to the water hole and then it's up to the horses to drink."
In other developments:
- A faction of the Sudan Liberation Army that did not sign last year's peace deal has reportedly agreed to a ceasefire and talks
- A UN human rights mission to Darfur has been denied visas, despite a promise otherwise from President Omar al-Bashir
- UN head Ban Ki-moon said the deteriorating situation in Darfur was unacceptable and he was still awaiting a reply from Khartoum on a UN peacekeeping force for Darfur.
Eastern Chad and Darfur have a similar ethnic make-up, with nomadic Arab groups and black African farmers both seeking access to land and scarce water points.
Our reporter says the violence in Chad follows the same pattern as in Darfur - mostly Arabs on camels and horseback attacking non-Arab villages.
Without an international protection force, there is no-one to stop the Janjaweed, she says.
In recent days, our reporter followed the trail of the Janjaweed through the ghost villages of eastern Chad, finding torched huts and smashed pots.
She met some of their victims, including a young man stabbed in both eyes and a frail old woman, badly beaten when she dared to look for food.
The UN Security Council is preparing to discuss proposals to send a peacekeeping force to Chad but a decision is not expected immediately.
1. Chad says Sudan government-backed militias are attacking villagers in Chad. Some 200,000 Darfur refugees are also in Chad
2. Sudan accuses Chad of backing the Darfur rebels
3. Chad says it will send troops to help CAR fight the rebels
4. CAR says Sudan backs rebels who have seized towns in CAR