Satellite images of Nigerian towns attacked by Boko Haram show widespread destruction and suggest a high death toll, Amnesty International says.
They show some 3,700 structures damaged or destroyed in Baga and Doron Baga last week, the human-rights group said.
Nigeria's government has disputed reports that as many as 2,000 were killed, putting the toll at just 150.
Amnesty cited witnesses saying that militants had killed indiscriminately. It said the damage was "catastrophic".
There has been a surge in violence linked to Boko Haram. In the past week there have been several attacks, including by suspected child suicide bombers.
Nigeria is to hold elections next month, amid growing doubts whether they can successfully go ahead in all parts of the country.
President Goodluck Jonathan made a surprise visit to the north-east on Thursday - the main focus of Boko Haram violence. It was his first trip to the region since March 2013.
In Maiduguri, capital of Borno State, he met survivors of the latest attack now living in a refugee camp.
"I want to assure you that you will soon go back to your houses," he said, according to the AFP news agency.
President Jonathan cancelled a trip to the region last year, leading to claims that he was indifferent to the fate of 200 schoolgirls kidnapped from the town of Chibok.
'Wiped off the map'
Amnesty said the attack on Baga and neighbouring Doron Baga, in the far north-east of Nigeria, was the largest and deadliest Boko Haram assault that it had analysed.
It said about 620 structures had been destroyed in Baga, and more than 3,100 in Doron Baga.
A businessman from Baga told the BBC how he fled from the town along with some 5,000 others.
He said people were being killed like animals.
Amnesty's before and after satellite images were taken on 2 January and 7 January. Healthy vegetation is shown in red on the graphics.
Many wooden fishing boats that dot the shoreline on 2 January are no longer visible five days later.
"These detailed images show devastation of catastrophic proportions in two towns, one of which was almost wiped off the map in the space of four days," Daniel Eyre, an Amnesty researcher, said in a statement.
"It represents a deliberate attack on civilians whose homes, clinics and schools are now burnt out ruins," he said.
The BBC's Will Ross says that while the images show the destructive nature of Boko Haram, they do not help establish just how many people were killed.
Last week, Musa Alhaji Bukar, a senior government official in the area, said that fleeing residents told him that Baga, which had a population of about 10,000, was now "virtually non-existent".
"It has been burnt down," he told the BBC Hausa service.
Boko Haram's capture of Baga
03 January: Social media reports of Baga attack first emerge
04 January: Boko Haram claims to have captured Baga
08 January: Reports emerge of bodies strewn on the streets in Baga, with some saying 2,000 people killed
12 January: The government says that the number of people who lost their lives in Baga was no more than 150, including militants
15 January: Satellite images released by Amnesty International suggest the number of dead is far higher than officially admitted
Officials said militants had attacked Baga on 7 January, four days after overrunning a multinational military base in the town that had been abandoned by Nigerian troops.
Amnesty's Adotei Akwei told the BBC that although it was still difficult to access the area where the attack took place, the Nigerian government was "grossly understating" the death toll.
"They killed so many people," one man told the group. "I saw maybe around 100 killed at that time in Baga. I ran to the bush. As we were running, they were shooting and killing."
Boko Haram at a glance
- Founded in 2002
- Initially focused on opposing Western education - Boko Haram means "Western education is forbidden" in the Hausa language
- Launched military operations in 2009 to create Islamic state
- Thousands killed, mostly in north-eastern Nigeria - also attacked police and UN headquarters in capital, Abuja
- Some three million people affected
- Declared terrorist group by US in 2013
One witness was quoted as saying that victims included small children and a woman in labour.
Afterwards, militants drove into the surrounding country, rounding up women, children and the elderly, unnamed witnesses told the group.
"Boko Haram took around 300 women and kept us in a school in Baga," one woman, who Amnesty said had been held for four days, was quoted as saying.
"They released the older women, mothers and most of the children after four days but are still keeping the younger women."