Nigeria's government on Wednesday denied reports of a mass kidnapping in the country's northeast, as Boko Haram militants flee a four-nation military offensive.

"There is no fresh kidnapping in Damasak," Nigeria's national security spokesman Mike Omeri told AFP, referring to the town recently retaken by forces from neighbouring Chad and Niger.

Reports suggested that the Islamist militants, who seized the town in Borno state earlier this year, made off with hundreds of children as they fled the troops' advance.

But Omeri said Nigeria had no information about a mass abduction. A senator who represents the area and a senior intelligence source also cast doubt on the reports.

The contradictory claims shed light on the difficulty of establishing facts in the brutal, six-year conflict, with communications infrastructure devastated in the northeast and travel restricted.

Officials, the military and locals frequently give contrasting information.

The militants do have a track record of mass kidnappings, however, including the high-profile abduction of more than 200 schoolgirls in April last year from the Borno town of Chibok.

Details about the kidnapping were muddied for weeks by military and official denials of details reported by the affected families.

The disputes over what actually happened were finally laid to rest when Boko Haram released a video picturing dozens of the hostages, who were subsequently identified by relatives.

Omeri noted Boko Haram's widely reported tactic of forcibly conscripting young boys during their hit-and-run attacks and attempts to indoctrinate them into the group's radical ideology.

Many Boko Haram fighters are believed to be on the run as a result of the offensive by Nigeria, Niger, Chad and Cameroon, who have routed the insurgents from dozens of towns.

Several sources said it was possible, and perhaps even likely, that scores of Boko Haram conscripts were missing, feared kidnapped by the militants across the region.

But they denied a specific mass abduction in Damasak, where the Chadian military last week said that about 100 bodies, some of them decapitated, were found in a mass grave.

Senator Maina Lawan, whose constituency includes Damasak, said: "I will be extremely surprised that such a huge number of my constituents would be abducted without me being informed.

"It is very unlikely that Boko Haram would have abducted such a huge number of people from Damasak because most of the people had fled months ago when Boko Haram took over."

A senior intelligence source in Borno's capital Maiduguri said there was "no iota of truth" to the mass abduction claims.