Northeast Nigeria is facing a 13-year armed insurgency by jihadist groups which has killed more than 40,000 people and displaced around two million from their homes.

The violence has spilled into neighbouring Niger, Chad and Cameroon, with the jihadists maintaining camps in the vast Lake Chad region straddling the four countries.

A Nigerian security source said there had an exodus of Boko Haram militants from Sambisa forest since last month due to a sustained bombing campaign on their hideouts.

Nigeria has also recorded a more intensive rainy season, which usually runs from May through September, and floods have occurred in almost every part of the country.

"The exodus of the Boko Haram terrorists has increased in recent days as the bombardments have intensified, coupled with the floodings that have submerged many of their camps," said the security source in the region who asked not to be identified.

On Monday, a convoy of more than 50 trucks carrying Boko Haram fighters and their families passed through villages on a route linking Sambisa with Lake Chad, several residents in the region said.

The fighters are believed to be loyal to Bakura Buduma, a Boko Haram factional leader, the sources said.

The convoy drove through Mafa forest into Jere and Koshobe before crossing between the towns of Gajiram and Gasarwa on the 135-kilometre (85-mile) highway linking the regional capital Maiduguri and the garrison town of Monguno, said the sources.

"They (Boko Haram) crossed the highway in batches of 10 vehicles at a time under the watch of heavily armed fighters," said Laminu Kontoma, a resident of the area.

After crossing the highway, the convoy moved into Gudumbali forest from where they emerged at Gaidam, before traversing a river into Abadam district on the border with Niger, said another resident, Bunami Garga.

"The Boko Haram convoy is definitely heading to the islands on Lake Chad in the Bosso area of Niger where the group has camps," said a fisherman named Kallah Sani who said he was familiar with Boko Haram movements in the region.

Niger authorities could not immediately confirm the movement.

- Infighting -

Those heading into Niger are Boko Haram fighters who had been holed up in parts of the Sambisa forest that remained under the group's control after it lost ground to a rival, the Islamic State West Africa Province (ISWAP).

ISWAP split from Boko Haram in 2016, rising to become a dominant jihadist group focusing more on attacking military bases and ambushing troops rather than civilians.

Boko Haram's leader Abubakar Shekau was killed in May 2021 during the infighting with ISWAP, which also seized most of the group's territory in Sambisa.

Some Boko Haram fighters moved out of Sambisa towards forests in the northwest where they forged alliances with criminal gangs involved in looting and kidnapping for ransom, according to a Nigerian intelligence report.