The Red Cross on Tuesday warned of an increasingly dire situation in Nigeria and other countries circling Lake Chad, where hundreds of thousands have been forced to flee Boko Haram violence.

"There is a full blown humanitarian crisis around this lake, not only in Nigeria but also in the surrounding countries," said Jean-Yves Clemenzo, spokesman for the International Committee of the Red Cross.

Troops from Chad and Niger have retaken the northeastern Nigerian town of Damasak from Boko Haram as part of a regional offensive to combat the militants who have pledged allegiance to the Islamic State group.

Boko Haram had held the town near the Niger border since November, part of a swathe of territory it had seized in Nigeria's northeast, where the group's six-year insurgency has been based.

The armies of Niger and Chad on Sunday launched a major ground and air strike against Boko Haram in northeastern Nigeria, opening a new front in regional efforts to eradicate the Islamist militants. 

The offensive announced by a source in the Niger government came after Boko Haram leader Abubakar Shekau pledged allegiance to the Islamic State group in an audio message.

The African Union has endorsed the creation of a regional force of up to 10,000 men to join the fight against the Nigerian Islamist group Boko Haram, diplomats said Friday.

The force, the idea for which was adopted at an AU summit in January, will be based in Chad's capital N'Djamena, the pan-African bloc's Peace and Security Council said.

Chadian President Idriss Deby on Wednesday vowed to "wipe out" Boko Haram and called on the group's chief Abubakar Shekau to give himself up, warning that he knew where the militant leader was hiding out.

"It is in Abubakar Shekau's interest to surrender, we know where he is. If he refuses to give himself up, he will suffer the same fate as his comrades," Deby said at a press conference with his visiting Niger counterpart.

Boko Haram gunmen attacked an island on the Niger side of Lake Chad, with locals reporting heavy casualties among those who fled but no official confirmation of the death toll.

A private radio station in Niger said that two people died in a fire in the remote village on Sunday evening but one survivor spoken to by Agence France-Presse said the death toll was higher.

Bomb attacks in Nigerian towns and cities look likely to increase in the run-up to forthcoming elections, despite the military claiming increasing successes against Boko Haram in captured territory.

At least 86 people were killed in explosions blamed on Boko Haram this week alone, all of them at crowded bus stations in the northeast, wider north and also in the country's central region.

A "ghost town" of burnt-out homes and looted properties greeted residents returning to Gamboru in northeast Nigeria for the first time since Chadian forces retook it from Boko Haram.

Scores of people crossed the 300-meter (yard) bridge that forms the border with Cameroon under military escort on Friday to survey the ravaged town.

Boko Haram militants suffered heavy casualties when Chadian troops pushed into Nigeria this week, residents who fled the fighting told Agence France Presse on Thursday.

Chad's army said on Tuesday evening that they had seized control of the town of Dikwa, which is about 50 kilometers (31 miles) southwest of the Nigerian border town of Gamboru.

Four African armies, from Nigeria, Cameroon, Chad and Niger, are currently directly engaged in the fight against the Nigeria-based insurgent group Boko Haram.

Regional forces have gone into action while awaiting the formation of a planned 8,700-strong, five-country force, whose make-up remains to be determined.