Area Senator Ahmed Zanna put the death toll at 300, citing information provided by locals, in an account supported by numerous residents.
Zanna said the town had been left unguarded because the soldiers based there had been redeployed north towards Lake Chad in an effort to rescue more than 200 girls kidnapped by Boko Haram on April 14.
The shocking mass abduction has sparked global outrage and offers of help from the United States, Britain, France and China.
Nigeria’s response to the kidnappings has been widely criticised, including by activists and parents of the hostages who say the military’s search operation has been inept so far.
The police on Monday offered 50 million naira ($300,000, 215,000 euros) for any information leading to the girls’ rescue.
President Goodluck Jonathan’s administration has sought to appear more engaged with the plight of the hostages in recent days, especially after Boko Haram chief Abubakar Shekau released a video threatening to sell the girls as “slaves.”
In a second kidnapping, eleven more girls aged 12 to 15 years were seized Sunday from Gwoza, an area not far from Chibok and also in Borno state, Boko Haram’s base.
The group’s five-year uprising has killed thousands across Africa’s most populous country and top economy, with many questioning whether Nigeria has the capacity to contain the violence.