Bodies lay strewn on the streets of a key north-eastern Nigerian town following an assault by militant Islamists, officials have told the BBC.
The Boko Haram group attacked Baga town on Wednesday, after over-running a military base there on Saturday, they said.
Almost the entire town had been torched and the militants were now raiding nearby areas, they added.
Boko Haram launched a military campaign in 2009 to create an Islamic state.
It has taken control of many towns and villages in north-eastern Nigeria in the last year.
The conflict has displaced at least 1.5 million people, while more than 2,000 were killed last year.
Nigerian lawmaker Maina Maaji Lawan said Boko Haram controlled 70% of Borno state, which is worst-affected by the insurgency.
Musa Alhaji Bukar, a senior government official in the area, said that fleeing residents told him that Baga, which had a population of about 10,000, was now “virtually non-existent”.
“It has been burnt down,” he told the BBC Hausa service.
Those who fled reported that they had been unable to bury the dead, and corpses littered the town’s streets, he said.
Boko Haram was now in control of Baga and 16 neighbouring towns after the military retreated, Mr Bukar said.
While he raised fears that some 2,000 had been killed in the raids, other reports put the number in the hundreds.
Mr Lawan, the senator for northern Borno, called on government troops to stop “dilly-dallying” and to fight back to protect residents.
“The indiscriminate killings went on and on and on,” he told BBC Focus on Africa.
Boko Haram’s offensive continued on Thursday, with its fighters setting up checkpoints and killing people who were hiding in the bush, the senator said.
Fleeing residents spoke of the stench of rotting corpses on the streets and surrounding bushes, he said.