Leading human rights groups on Thursday released satellite images claiming to show massive destruction by Boko Haram of two Nigerian towns in what is feared to be the deadliest strike of the Islamists' six-year insurgency.
Amnesty International and Human Rights Watch published separate images of Baga and the nearby town of Doron Baga, on the shores of Lake Chad, in the far north of Borno State in northeast Nigeria.
Hundreds of people, if not more, are feared to have been killed in the attack, Amnesty said, that is thought to have targeted civilian vigilantes helping the army and that reportedly included one woman being killed while in labour.
But Nigeria's military, which often downplays death tolls, said that 150 died and dismissed as "sensational" claims that 2,000 may have lost their lives.
HRW said the exact death toll was unknown and quoted one local resident as saying: "No one stayed back to count the bodies.
"We were all running to get out of town ahead of Boko Haram fighters who have since taken over the area."
Amnesty's images showed aerial shots of the two towns, which have been hit previously by fighting, on January 2 -- the day before the attack -- and January 7, after homes and businesses were razed.
The group said the images suggested "devastation of catastrophic proportions", with more than 3,700 structures -- 620 in Baga and 3,100 in Doron Baga -- damaged or completely destroyed.
HRW said 11 percent of Baga and 57 percent of Doron Baga was destroyed, most likely by arson, attributing the greater damage in the latter to the fact that it houses a regional military base.
The Multinational Joint Task Force of troops from Nigeria, Niger and Chad has been involved in counter-insurgency operations against Boko Haram.
At least 16 settlements around Baga were burnt to the ground and at least 20,000 people fled, according to local officials.
Harrowing testimony has been emerging from survivors about the scale and brutality of the assault in Baga.
Eye-witnesses spoken to by AFP described seeing decomposing bodies in the streets and one man who escaped after hiding for three days said he was "stepping on bodies" as he fled through the bush.
Amnesty said on Thursday that survivors have told them that Boko Haram fighters killed a woman as she was in labour, during indiscriminate fire that also cut down small children.
"Half of the baby boy (was) out and she died like this," the unnamed witness was quoted as saying.
A man in his fifties added: "They killed so many people. 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."
Another woman said: "I don't know how many but there were bodies everywhere we looked."
Medical charity Doctors Without Borders (MSF) said on Tuesday that its team in the Borno State capital, Maiduguri, was providing assistance to 5,000 survivors of the attack.
The UN refugee agency has said that more than 11,300 Nigerian refugees have fled into neighbouring Chad.
Amnesty said the eye-witnesses 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 northeast Nigeria.
"The deliberate killing of civilians and destruction of their property by Boko Haram are war crimes and crimes against humanity and must be duly investigated," it added.
Some 300 women were said to have been rounded up and detained at a school, witnesses told Amnesty, adding that older women, mothers and children were released after four days but younger women kept.
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.
Nigeria's electoral commission said voting was "unlikely" in rebel-controlled areas and arrangements were being made to allow hundreds of thousands of displaced people to cast their ballots.