"The attack on Baga and surrounding towns looks as if it could be Boko Haram's deadliest act," Amnesty International said in a statement.
Islamist militants sprayed bullets as they stormed in last weekend in trucks and armored vehicles, local authorities said Friday.
When they arrived, they unloaded motorcycles and pursued residents who fled into the bush, firing indiscriminately, said Baba Abba Hassan, a local district head.
Local officials reported death tolls ranging from hundreds to as many as 2,000 people.
"Dead bodies litter the bushes in the area and it is still not safe to go and pick them (up) for burial," said Musa Bukar, the chairman of the local government where Baga is located.
"Some people who hid in their homes were burned alive."
Raid lasted for days
During the raid that started January 3, hundreds of gunmen seized the town of Baga and neighboring villages, as well as a multinational military base.
Attacks started at dawn and continued throughout last weekend, according to residents.
Though local officials gave conflicting death tolls, they agreed on the massive number of fatalities.
More than 2,000 people were killed in attacks on 16 villages, Bukar said. He could not explain how he arrived at that toll.
But the local district head said hundreds of people had been killed, not thousands. The actual toll will be known after a headcount of households is complete, Hassan said.
An offensive is underway to reclaim the areas from the militants, according to Mike Omeri, a government spokesman.
Tens of thousands displaced
At least 30,000 people were displaced, authorities said. About 20,000 of the displaced camped in Maiduguri city, the capital of Borno state.
Authorities are making arrangements to transport the 10,000 others from Monguno town, 60 kilometers (36 miles) from Baga. Some residents fled into neighboring Cameroon and Chad.
"If reports that the town was largely razed to the ground and that hundreds or even as many as 2,000 civilians were killed are true, this marks a disturbing and bloody escalation of Boko Haram's ongoing onslaught against the civilian population," Amnesty International's Daniel Eyre said.
Boko Haram has terrorized northern Nigeria regularly since 2009, attacking police, schools, churches and civilians, and bombing government buildings. It has also kidnapped students, including more than 200 schoolgirls who were abducted in April and remain missing.
The Islamist group has said its aim is to impose a stricter form of Sharia law across Nigeria, which is split between a majority Muslim north and a mostly Christian south.
The United States condemned the attacks, saying the group "shows no regard" for human life.
"All those responsible for these recurring terrorist attacks must be held accountable," State Department spokeswoman Jen Psaki said.
Threats against Cameroon
In neighboring Cameroon, President Paul Biya urged the international community to help battle the terror group. His call came after the leader of the terrorist group threatened him in a YouTube video.
"Oh Paul Biya, if you don't stop this, your evil plot, you will taste what has befallen Nigeria. Your troops cannot do anything to us," Boko Haram leader Abubakar Shekau said in the video released this week.
The threat against the President came after Cameroonian soldiers killed dozens of Boko Haram fighters this month.
Biya told the international community that a global response is required to combat the terror group.