"... initial reports indicate that as many as 220 people have been killed in this week's string of attacks, with dozens wounded," the group said in a statement.
Humanitarian workers in Chad said the attacks were carried out by gun-toting Arab tribesmen against African villagers often armed with nothing more than bows, arrows and swords.
"Around 200 men on horseback attacked, accompanied by two Toyota pick-ups," a humanitarian worker in touch with colleagues in the area said about one of the attacks, which took place around Dar Sila in Chad's eastern province of Ouaddai.
"The attackers shouted 'You slaves! We have arrived and now we are attacking you'," added the worker.
Arab tribes of the Sahara enslaved black Africans for centuries.
Chad's Territorial Administration Minister Ahmat Mahamat Bachir was visiting the Dar Sila area, around 40 km south of the eastern Chadian town of Goz-Beida to investigate the clashes.
"I don't have a precise number of dead. ... I know it's more than 100," he told Reuters by phone.
The attacks followed clashes last week between Arabs and non-Arabs in Salamat province, south of Ouaddai, in which over 100 people were killed, triggering calls for UN peacekeepers to deploy in Chad and neighbouring Central African Republic.
Aid workers in Chad said the attackers burnt the village of Djorlo to the ground. The raiders were drawn from three local Arab ethnic groups and targeted villages of the non-Arab Dadjo and Moro tribes, they said.
The pattern of violence reflects that of Darfur, where government-backed mounted Arab militia known as Janjaweed have attacked villagers and burnt homes in a war with rebels that has killed tens of thousands of people since 2003.