"What they have done... Is a crime against humanity, nothing less," Kerry said as first images of what is feared to be the worst atrocity of the six-year Islamist insurgency emerged.
Hundreds of people, if not more, are reported to have been killed in attacks on the towns of Baga and Doron Baga on the shores of Lake Chad in Borno state, according to Amnesty International.
Amnesty and New York-based watchdog Human Rights Watch published separate satellite images Thursday claiming to show massive destruction in the adjacent towns, adding to fears they may suffered the deadliest strike yet in Boko Haram's bloody campaign.
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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 per cent of Baga and 57 per cent of Doron Baga was destroyed, most likely by fire, attributing the greater damage in Doron Baga to the fact that it houses a regional military base.
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 in the attacks.
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 said Boko Haram were believed to have targeted civilian vigilantes helping the army after they overran a Multinational Joint Task Force base for troops from Nigeria, Niger and Chad who have been involved in operations against them.