The 86 elephants, included 33 pregnant females and 15 calves, conservation groups said on Tuesday.
The killing was the worst in the region since more than 300 elephants were slaughtered in Cameroon early last year. Both raids took place during the dry season when poachers armed with automatic weapons launch coordinated attacks on herds of elephants in the region.
Conservationists warn that organised criminal gangs are illegally trafficking huge quantities of tusks to cash in on soaring demand for ivory in Asia.
The attack was reported to have taken place on March 14 to 15 in southern Chad, near the border with Cameroon.
"This tragedy shows once again the existential threat faced by Central Africa's elephants," Bas Huijbregts, head of the World Wildlife Fund's [WWF] campaign against illegal wildlife trade in Central Africa, said in a statement.
Citing local officials, WWF said the poachers were on horseback and spoke Arabic, suggesting that they were the same group who had been involved in the March 2012 attack that killed more than 300 elephants in northern Cameroon.
Faced with mobile and heavily armed poaching teams, Cameroon has deployed military helicopters and hundreds of troops to some national parks to protect the animals.
The International Fund for Animal Welfare (Ifaw) confirmed the attack, saying the elephants' tusks had been hacked out. It said elephant populations in the region risked being wiped out.
"The killing of 86 elephants, including pregnant cows, is evidence of the callous brutality demanded to feed the appetite of the ivory trade," said Celine Sissler-Bienvenu, head of Ifaw in France and Francophone Africa.
Demand for ivory for use in jewellery and ornamental items is rising fast in Asia. Conservationists say growing Chinese influence and investment in Africa has opened the door wider for the illicit trade in elephant tusks.
"Cross-border cooperation and intelligence-led enforcement are the only ways we can bring these ivory traffickers to justice. It is too big a problem for any one country to tackle," said Kelvin Alie, director of Ifaw's Wildlife Crime and Consumer Awareness Programme.
"We need range states, transit countries and destination countries to share their law enforcement resources, including intelligence, or we'll never be in a position to shut down the kingpins of the international ivory trade," Alie said.
Data collected by conservationists shows that killing rates for elephants in Africa have risen dramatically in recent years.
From about 11 500 elephants illegally killed in 2010 in areas observed by the Monitoring Illegal Killing of Elephants programme, estimates for 2011 and 2012 rose to around 17 000. – Reuters