Amnesty International said in statement released late on Friday: "Over the last 10 days, close to a dozen villages in eastern Chad have been attacked, with at least 40 people killed."
The London-based organisation said that, according to information it had obtained, about 3,000 villagers in eastern Chad had fled their homes in the past week either because their village had been attacked or because they expected it to be attacked.
The statement said: "Fleeing villagers described the attackers as Janjaweed wearing Sudanese army uniforms."
Hourmadji Moussa Doumgor, a Chad government spokesman, said on Saturday that he had no information about the fighting.
Attacks on villagers
Kate Gilmore, the executive deputy secretary general of Amnesty International, said in the statement: "We warned that these attacks were likely to resume once the rainy season ended, and now they have started, as we predicted.
"These attacks could have been avoided if the Chadian government and the international community had listened to earlier warnings and acted in advance to protect civilians in eastern Chad."
The Janjaweed fighters from Sudan's western region of Darfur have in the past been accused of attacking villages in eastern Chad because some of the villagers are members of the same tribes as ethnic Africans in Darfur, where African rebels rose against the Arab-dominated central government in 2003.
Sudan has been accused of unleashing a fierce military repression following the uprising and of letting the Janjaweed commit atrocities against African villagers in Darfur.