Since Friday, Chadian fighter jets have been bombing Boko Haram camps on remote islands in the vast marshland shared by Nigeria, Cameroon, Niger and Chad.

The bombardment has also killed dozens of Nigerian fishermen working on islands under Boko Haram control, where civilians are forced to pay taxes to the jihadist group.

Footage seen by AFP shows several fishermen with severe burns being treated at a hospital in Bosso, Niger.

"Boko Haram have been fleeing islands in Shuwa area on the boundary between Nigeria, Niger and Chad," fisherman Suleiman Hassan told AFP.

Hassan arrived in Maiduguri, the capital of Nigeria's Borno state, on Monday after escaping the area with other fishermen in a small boat.

"Boko Haram are fleeing their camps in several islands with their families in small boats under bombardment," Hassan said, naming Dogon Chukwu, Kangarwa, Gashakar, Yawan Mango, and Kwatar Mota islands

Sources said Chadian troops also clashed with Boko Haram fighters on Kaukeri island, the group's main bastion on Lake Chad.

The strikes are widely seen as retaliation for recent deadl attacks by Boko Haram on the Chadian military. Last week, Chad declared three days of mourning after an ambush on an army patrol killed two generals.

Two days earlier, a raid on a military base along the lake's shores killed at least 24 Chadian soldiers.

A regional intelligence source confirmed that Nigeria and Niger have joined the aerial campaign.

"The air strikes are being coordinated by Chad, Nigeria and Niger, with each of them providing two fighter jets for the operation," the source said, requesting anonymity.

They added that Chadian troops had launched ground assaults on Kangallam and Shuwaram islands, "two (of) Boko Haram's major enclaves".

"They reached the two islands by boats and engaged the terrorists in a fierce fight," the intel source said.

They said the fleeing jihadists were now stranded on the shores of the Nigerian side of the lake, afraid to move further into areas where ISWAP, a rival group that split from Boko Haram in 2016, has camps.

Boko Haram had previously pushed ISWAP off most islands in the lake, forcing the group to establish bases along its fringes.

The jihadist insurgency that erupted in 2009 has killed several thousand people and displaced millions in northeast Nigeria alone.

In recent years, the violence has spilled into neighbouring Niger, Chad and Cameroon, prompting a regional military coalition to fight the jihadist groups.

The alliance had suffered setbacks in recent years after Niger withdrew following the military takeover in the west African country.