"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 andDoron Baga on the shores of Lake Chad in Borno state, according to Amnesty International.
Boko Haram was "evil" and a serious threat "not just in Nigeria and the region but to all of our values", Kerry said during a visit to Bulgaria. He said he had spoken earlier to his British counterpart Philip Hammond -- who was also in Sofia -- about the possibility of "a special initiative with respect to Nigeria and with respect to Boko Haram".
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.
Amnesty's images showed aerial shots of the towns 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 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.
Local officials have said at least 16 settlements around Baga were burnt to the ground and that at least 20,000 people fled.
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.
'Killed in labour'
Harrowing testimony has been emerging from survivors about the scale and brutality of the assault in Baga, included one woman reportedly killed while in labour.
Witnesses who spoke to 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 it had received accounts from survivors of Boko Haram fighters killing a woman as she was giving birth, 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 capital of Borno state, Maiduguri, was providing assistance to 5,000 survivors of the attack.
The UN refugee agency has said that more than 11,300 Nigerian refugees fled into neighbouring Chad.
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.
Amnesty said the witness accounts and images reinforced fears the attack was Boko Haram's "largest and most destructive" in its fight to establish a hardline Islamic state in northeast Nigeria, which has killed over 13,000 people since 2009.
"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.
The Baga attack came before presidential and parliamentary elections in Nigeria next month and an upsurge in violence apparently designed to undermine 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.