Amnesty International has released satellite images of the towns of Baga and Doron Baga in north-eastern Nigeria, showing what it says is shocking evidence of last week's attacks by Boko Haram militants.
The organisation said the images - from before and after the attacks - showed more than 3500 structures destroyed and matched the harrowing stories of eyewitnesses.
Amnesty International's Adotei Akwei said accurate information was hard to come by because of the remoteness of the region and the danger there, the BBC reported.
But he said an official Nigerian figure of 150 dead was a major underestimation.
"It is a remote area. It is extremely difficult and at the moment it is too dangerous to actually get back into Baga," he said.
"But Amnesty has been in contact with journalists and has been in contact with some of the survivors, and we believe the Nigerian government is grossly understating the numbers of casualties.
"We believe the number will be closer to 2000 and a far cry from 150."
Daniel Eyre, the Nigeria researcher for Amnesty International, said eyewitnesses described indiscriminate shooting and killing.
Mr Eyre said one eyewitness described stepping on bodies for more than five kilometres as he fled through the bush.
He said the accounts and images reinforced the view that the attack was Boko Haram's "largest and most destructive" in its fight to establish a hardline Islamic state in north-east Nigeria.
Thousands flee following attacks
Nigeria's military, which often downplays death tolls, said this week that 150 people died, dismissing as "sensational" claims that 2000 may have lost their lives, AFP reported.
Security analysts said it may be impossible to know exactly how many were killed, with the town and surrounding area still in rebel control.
Local officials have said Baga and at least 16 surrounding settlements were burnt to the ground and at least 20,000 people fled.
Medical charity Doctors Without Borders said on Tuesday (local time) that its team in Borno State capital Maiduguri was providing assistance to 5000 survivors of the attack.
The UN refugee agency has said that more than 11,300 Nigerian refugees have fled into neighbouring Chad.
The Baga attack came before presidential and parliamentary elections in Nigeria next month and an upsurge in violence apparently designed to undermine the legitimacy of the vote.
On Saturday, 19 people were killed when explosives strapped to a young girl said to be as young as 10 detonated at a crowded market in Maiduguri.
Four people were also killed when two female suicide bombers hit another market in the commercial capital of neighbouring Yobe State, Potiskum, on Sunday.
- BBC / AFP