About 500 children aged 11 and under are missing from a Nigerian town recaptured from militants, a former resident of Damasak has told the BBC.
A trader in the north-eastern town told Reuters news agency that Boko Haram fighters took the children with them when they fled.
Troops from Niger and Chad seized Damasak earlier in March, ending months of control by the Islamist militants.
A regional force has recently been helping Nigeria take on the insurgents.
Thousands have been killed since 2009, when Boko Haram began its insurgency to create an Islamic state.
The senator representing the north of Borno state, Maina Maaji Lawan, told the BBC's Nigeria correspondent, Will Ross, that the case in Damasak was typical and many hundreds of children were missing.
He said: "The very young ones they give to madrassas [Islamic schools]… and male ones between 16 and 25, they conscript them and they indoctrinate them as supply channels for their horrible missions."
Boko Haram caused international outrage in April 2014 after it abducted more than 200 girls from a boarding school in Chibok town in north-eastern Nigeria's Borno state.
The group's leader Abubakar Shekau has said the girls have been married off.
Damasak is a trading town in Borno state near Niger's border and is about 200km (120 miles) from the state's main city of Maiduguri.
It was overrun by the militants at the end of last year.
Damasak businessman Malam Ali, whose brother is among those missing, told the BBC Hausa Service that young boys had been put in a madrassa by Boko Haram when they took over the town.
Following the recapture of the town, those boys had not been accounted for, he said.
Our correspondent says the conflict has torn many families apart.
As towns have changed hands it has been impossible to work out how many people have been killed and how many are missing, he adds.
Last week, the decomposing bodies of more than 70 people were discovered under a bridge near Damasak.
It is widely believed that these were civilians killed by the militants, our correspondent says.
Most of the areas which Boko Haram had seized have now been recaptured, Nigeria's army says.
Boko Haram at a glance:
- Founded in 2002, initially focused on opposing Western-style education
- Boko Haram means "Western education is forbidden" in the Hausa language
- Launched military operations in 2009 to create an Islamic state
- Thousands killed, mostly in north-eastern Nigeria
- Has also attacked police and UN headquarters in capital, Abuja
- Abducted hundreds, including at least 200 schoolgirls
- Pledged allegiance to Islamic State