Boko Haram fighters were patrolling the streets of Bama, preventing people from burying the dead, Ahmed Zanna said.
|Boko Haram has always had a scorched earth policy|
(The Army had actually repelled the first attack).
Officials said about 26,000 people had been displaced by fighting in Bama, a key town in the battle for control of Nigeria's north-eastern Borno state.
Earlier this week, the Nigeria Security Network (NSN) think-tank said the group had made "lightning territorial gains" in recent months, raising fears that the country could disintegrate like Syria and Iraq, where the Islamic State (IS) rebel group has declared a caliphate.
Boko Haram has also said it has set up a caliphate in the areas it controls - it is not clear if the two groups are allied.
Mr Zanna, a senator in Borno, said the humanitarian situation in Bama was "terrible" and there had been a "lot of killings" in the town.
|Since there is very little to the north of Maiduguri,|
it is nearly surrounded by Boko Haram
Boko Haram has captured a string of towns in northern-eastern Nigeria in recent months, fuelling concern that it could advance towards the main city, Maiduguri.
Mr Zanna said it would be "catastrophic" if Boko Haram launched an assault on Maiduguri, which has a population of more than two million.
"I'm begging the government to send more troops and armoury to Maiduguri," he said.
"Boko Haram do come overwhelmingly because they recruited en masse in the villages [in Borno state]," he added.
|Abubaker Shekau Boko Haram leader|
Residents told BBC Hausa that Boko Haram returned to the town on Tuesday with reinforcements after being repelled by government forces the previous day.
On Wednesday, Borno state deputy governor Zannah Umar Mustapha denied the militants had taken over Bama, which had a population of about 270,000.
He told the BBC Hausa service that the army was still fighting them.
The government's National Emergency Management Agency (Nema) said on Wednesday that 26,391 had been displaced by the fighting.
"The number is growing by the hour," its spokesman Abdulkadir Ibrahim told Reuters news agency.