Kano (Nigeria) (AFP) - Boko Haram gunmen killed dozens of soldiers and massacred civilians in a weekend attack on an island on Lake Chad, after 21 people were shot dead in northeastern Nigeria.

The region has been relentlessly targeted by Boko Haram throughout the jihadist group's six-year uprising, but there has been a lull in violence in recent weeks.

A coalition of troops from Chad, Cameroon, Niger and Nigeria has claimed major victories over the rebels since February, reportedly flushing the militants out of areas they previously controlled.

But the latest unrest highlighted the continuing and grave threat posed by Boko Haram, an extremist group which has killed at least 15,000 people and forced 1.5 million from their homes.

- Island attack -

Islamist fighters travelling in motorised canoes stormed the island of Karamga on Lake Chad before sunrise on Saturday.

Troops from Niger stationed on the island "were caught off guard" and suffered heavy losses, said Umar Yerima, a fisherman who witnessed the raid but managed to escape.

Niger's military has confirmed the attack but defence officials were not immediately available to provide a death toll.

A security source in Chad who requested anonymity said Niger lost 48 soldiers and another 36 were missing.

Another source, an official from southeastern Niger's Diffa region, said the toll was much higher at 80 dead and some 30 missing.

Yerima told AFP the Islamic State group-allied militants "turned their guns" onto civilians after overpowering the troops.

"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' (God is greatest)," he added.

"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 sight by hiding in the long grass that lines the water's edge.

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The gunmen remained on a rampage until about roughly midday (1100 GMT), withdrawing when a military jet started bombarding the area, according to Yerima, who estimated the civilian death toll was "huge" but could not provide specifics.

The security source in Chad said an aerial bombardment on the island was carried out by Chadian helicopter gunships based in Diffa.

- 'Returnees' killed -

Suspected Islamists disguised as soldiers on Friday intercepted a group of people trying to return to their homes to collect abandoned food supplies in northeastern Nigeria's Yobe state.

"The men, 21 of them, were stopped at Bultaram (village) by gunmen we believe are Boko Haram who shot them dead," said Baba Nuhu, an official with the Gujba local government in Yobe state.

Nuhu and Haruna Maram, the brother of one of the victims, spoke to AFP from Yobe's capital Damaturu, where many Gujba residents have sought refuge from the violence.

"My brother and 20 others wanted to bring back their grains to augment their lean food supplies," Maram said. "Unfortunately, they were killed by (the) same Boko Haram we ran away from."

Gujba is one of a handful of districts in Yobe that Boko Haram captured during its sweeping offensive last year.

It was also the scene of a gruesome massacre at an agricultural college in 2013 that targeted students sleeping their dormitories.

- Warning -

Following the purported victories secured in the joint offensive, Nigeria's military and outgoing President Goodluck Jonathan have encouraged those displaced by the uprising to return home.

But community leaders in the embattled region have warned civilians are still at risk, especially those returning to remote areas like Gujba where the military's deployment has typically been thin.

Many experts agree that Boko Haram is likely to transition from capturing and holding territory -- a tactic seen from mid-2014 -- and revert to a pattern of hit-and-run attacks.

Analysts say boastings from regional armies should be treated with caution, as the Islamists are far from defeated and can easily regroup, especially if there is a let-up in the military pressure.

Nigeria's president-elect Muhammadu Buhari, an ex-army general, takes office on May 29. He has vowed to fight Boko Haram more effectively than Jonathan.