Nigeria's government may have negotiated a supposed ceasefire with only one faction of Boko Haram, an influential regional group has said, as violence continued unabated in the far northeast.

The Borno Elders Forum, made up of retired senior civilian and military officials from the state, said attacks in recent days indicated that not all Boko Haram fighters were aware of the deal announced on Friday.

"If they are aware and they are in agreement that there is a ceasefire, I don't think they would continue attacking innocent people and taking over places," said the elders' spokesman Bulama Mali Gubio.

"It is either (that) those the federal government is negotiating with are not the Boko Haram but the usual 419ers... or it is just some kind of mockery," he told reporters in Maiduguri on Sunday evening.

The term 419 refers to the section of the Nigerian penal code that deals with fraud and which has become a catch-all for con artists.

Gubio was speaking after senior military and intelligence officials in the violence-wracked region told AFP at the weekend that they had not been informed of the ceasefire.

There was a wave of attacks in Borno and Adamawa states, including one on Sunday evening in Damboa, southwest of Maiduguri, where soldiers fought the militants.

A senior military source, speaking on condition of anonymity, said on Monday that troops killed at least 35 insurgents as they tried unsuccessfully to ransack the town.

Rocket-propelled grenades, machine guns and explosives were recovered after the fighting, which lasted from 7:00 pm on Sunday (1800 GMT) to the early hours of Monday.

Separately, witnesses in northern Borno reported that a significant number of people fleeing weekend clashes drowned while trying to cross the Kumadugu-Yobe river that serves as a border with neighbouring Niger, but casualty figures were not immediately clear.

Usman Alhaji, a resident of the embattled border town of Abadam, said barge operators that carry people into Niger stopped work because of the violence.

Some, "especially women and children" drowned as they tried to swim to safety, he said, in an account supported by other witnesses who spoke to reporters after reaching Maiduguri.

The UN refugee agency estimates that more than 60,000 people have fled across Nigeria's northeastern borders trying to escape Boko Haram attacks.

The Borno Elders Forum, which last month warned that Boko Haram had surrounded Maiduguri and was preparing to take over, suggested Abuja had not negotiated with the entire group.

"If the federal government does not know who the real Boko Haram are, I think they should come here to find out from us," Gubio said.

"The real Boko Haram who are killing us, who are burning our towns and villages, are not the Boko Haram that a peace deal was reached with."

The government claimed a deal was reached after talks in Chad, which included provisions for the release of more than 200 schoolgirls kidnapped from the Borno town of Chibok in April.

But no detail of the terms and conditions for Boko Haram to lay down their arms was given and many observers expressed doubts about the identity of the group's supposed envoy.

Gubio said the deal was "very sketchy" and questioned whether it meant that each side would retain its positions, which for Boko Haram would mean the towns it has captured in recent months.

An analysis by Nigerian Counter-insurgency (nigeriancoin) suggested that Boko Haram was in control of 20 towns in Yobe, Borno and Adamawa states with pre-insurgency populations of more than 10,000 as of last Friday.

The towns include the Borno town of Gwoza, home to nearly 50,000 people, which Boko Haram declared last month to be part of an Islamic caliphate.

The military has claimed to have taken back five towns but residents maintain the occupation is ongoing, according to information from the open source analysis website nigeriancoin.wordpress.com.