Boko Haram gunmen massacred residents who tried to jump into Lake Chad seeking safety and burnt others alive after overpowering soldiers in a weekend attack in the restive region, a witness said Monday.
The military and a local official in Niger had previously confirmed the April 25 raid by the Islamists on Lake Chad's Karamga island.
An official in the town of Diffa said the Nigerien army suffered "very heavy" casualties, but precise figures were not immediately available.
Umar Yerima, a Nigerian fisherman living on the targeted island confirmed that troops "were caught off guard" and suffered major losses.
"After finishing with the soldiers, (the Islamists) turned their guns on residents," Yerima told AFP by phone, adding that he was "among the lucky ones" who managed to flee.
"Some sought to escape by plunging into the lake but gunmen stood on the shore shooting them...
"They would aim their gun from the edge of the lake and shoot any head that emerged from the water, shouting Allahu Akbar," he further said.
"They burnt the entire village and went on a shooting spree. Many residents were burnt alive in their homes."
Yerima said he managed to stay out of site by hiding in the long grass that lines the water's edge.
After launching the attack shortly before sunrise, the gunmen remained on a rampage until roughly midday, withdrawing when a military jet began bombarding the area.
It was impossible to estimate the death toll, Yerima told AFP, but said he believed the number was "huge" and that he saw the Islamists kidnap a number of women and children as they fled.
Boko Haram has been hit hard by a four-nation offensive launched in February by Nigeria, Niger, Chad and Cameroon, with much of the fighting concentrated in the Lake Chad area.
While the militaries involved have claimed major successes, including the recapture of a series of towns and villages held by the Islamist rebels, experts believe Boko Haram fighters have carved out new hiding spots in remote enclaves.
The Nigerien military said the Islamists stormed Karamga on motorised canoes after setting off from a base elsewhere on the lake.
The Boko Haram conflict has killed more than 13,000 people since 2009, mostly in northern Nigeria, but the fighting has increasingly spread
to neighbouring states since 2013.
Despite the apparent gains made the multi-national offensive, security experts say Boko Haram has the capacity to regroup and have urged the region's forces to maintain military pressure.
Nigeria's president-elect Muhammadu Buhari takes office on May 29 and has vowed to fight Boko Haram more effectively than the country's outgoing leader, Goodluck Jonathan.