A Nigerian fighter jet early on Sunday bombarded Kwatar Daban Masara in Lake Chad, which straddles Nigeria and neighbouring Niger, Chad and Cameroon, the sources said. The area is a bastion for the IS-affiliated Islamic State West Africa Province (ISWAP).

The reports of casualties came less than two weeks after officials say another air strike on a village killed nine civilians in Nigeria's northeast where the military is battling a 12-year Islamist insurgency.

ISWAP recently lifted a ban on fishermen in its territory, allowing them to move in and fish in the freshwater lake for a fee. That led to an influx of fisherman who had abandoned the area.

"Any fisherman that goes to that area does so at his own risk because it is an enemy territory and there is no way of differentiating them from the terrorists," one local intelligence source said.

"From the information we have been receiving the death toll is much higher than 20."

Local fisherman Labo Sani told AFP by telephone that the jet strike hit Kwatar Daban Masara at around 6 am (0500 GMT), killing around 20 fishermen.

Kwatar Daban Masara which is on the shores of the lake is the "gateway to ISWAP's camps on several islands" and a supply route for the jihadists, said Sani from nearby Daban Masara.

Another fisherman, Sallau Arzika, said the fighter jet struck the village "killing many of our people who are there for fishing".

"The initial death toll was around 20, but the figure has been increasing with the deaths of many of the injured," Arzika said.

News of the incident was slow the emerge due to limited telecommunications in the region.

A Nigerian air force spokesman did not immediately respond to a message seeking confirmation or comment.

The intelligence source who works with anti-jihadist militia in the region said the strike was based on "credible information" of a gathering of ISWAP fighters in the village since Wednesday.

He said aerial surveillance and reports from other sources revealed terrorists were amassing in Kwatar Daban Masara and it was obvious they were planning an attack, the source said.

- 'Preemptive strike' -

According to another security source, the village was subjected to surveillance in the past 10 days after scores of men suspected to be foreign fighters arrived in several vehicles and were ferried to camps inside the lake.

"It was a preemptive strike to destroy whatever plans the terrorists were making," said the security source.

"You don't expect an innocent civilian to be at that location and whoever is found is certainly part of the terrorists," he said.

On September 16 a Nigerian airstrike on a village in nearby Yobe state killed at least nine civilian residents, according to officials.

The Nigerian air force said at the time its fighter jet was pursuing a group of jihadists in the area and it was investigating the incident.

Since 2009, Nigeria's armed forces have battled a grinding jihadist insurrection in the northeast that has killed more than 40,000 people and displaced nearly two million from their homes.

The conflict spilled into neighbouring Niger, Chad and Cameroon, prompting a regional military coalition to end the violence.

In January 2017 at least 112 people were killed when a fighter jet struck a camp housing 40,000 people displaced by violence in Rann near the border with Cameroon.

The Nigerian military blamed "lack of appropriate marking of the area" for the bombardment in a report it issued six months later.

In July 2019 at least 13 civilians were killed when a Nigerian fighter jet hit Gajiganna village, 50 kilometres from Borno state capital Maiduguri, as it targeted fleeing jihadists after they attacked a nearby base.