Boko Haram fighters killed a woman as she was in labour during what is feared to be the deadliest attack in the militants' six-year insurgency, Amnesty International claimed on Thursday.
The human rights group said one witness to the assault on Baga, on the shores of Lake Chad in northeast Nigeria, told them the woman was shot by 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.
Amnesty said this week that hundreds of people, if not more, may have been killed in the attack, which began on January 3 and is thought to have targeted civilian vigilantes helping the military.
"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," a man in his fifties was quoted as saying.
Another woman added: "I don't know how many but there were bodies everywhere we looked."
The testimony chimes with claims from local officials that huge numbers were killed and that of witnesses spoken to by AFP, who described seeing decomposing bodies littering the streets.
One man who escaped from Baga after hiding for three days said he was "stepping on bodies" for five kilometres (three miles) as he fled through the bush.
Nigeria's military, which often downplays death tolls, said this week that 150 people died, dismissing as "sensational" claims that 2,000 may have lost their lives.
Human Rights Watch said the exact death toll was unknown and in a statement published on Thursday 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."
Both Amnesty and HRW published separate satellite images of Baga and Doron Baga, 2.5 kilometres away, which it said showed the scale of the attack.
Aerial shots of the two towns - which have been hit previously by fighting - were shown the day before the Islamists moved in and four days later, after they had razed homes and businesses.
Amnesty said that the images showed "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% of Baga and 57% of Doron Baga was destroyed, most likely by arson, attributing the greater damage in the latter to the fact that it houses a 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.
Medical charity Doctors Without Borders 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.