Boko Haram Nigeria
Soldiers from Niger patrol in a truck at Kabalewa Refugee Camp, where Nigerians fleeing from Boko Haram are sheltered Olatunji Omirin/AFP

An unconfirmed number of civilians have been killed by terror group Boko Haram during an attack at Lake Chad's Karamga island at the weekend.

Witnesses said that the island's fishermen and troops were "caught off guard" by the terrorists, who reached Karamga on motorised canoes.

"After finishing with the soldiers, [Boko Haram] turned their guns on residents," fisherman Umar Yerima told AFP, adding that he was "among the lucky ones" who managed to flee.

Boko Haram Nigeria

Who are Boko Haram terrorists?

Boko Haram fights against Western influence in Nigeria and aims to impose its version of Sharia law in the country. The group declared an Islamic caliphate in Gwoza, along the Cameroon border, in August 2014.

Boko Haram has raided several cities in the north of the country in a bid to take control of more land.

Three states, Adamawa, Borno and Yobe, have been under a state of emergency since May 2013, due to Boko Haram's deadly attacks (pic: Reuters)

"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 continued.

"They burnt the entire village and went on a shooting spree. Many residents were burnt alive in their homes."

The weekend attack came as Nigerian troops, aided by soldiers from Chad, Benin, Cameroon and Niger, are regaining several territories previously controlled by the terrorists, as part of a major ground and aerial offensive that started in March.

Following the latest raid, Nigeria's President Muhammadu Buhari deemed the terrorists a "fraud" who have nothing to do with religion.

"The fraud called Boko Haram can be defeated by denying it a recruitment base. No religion allows for the killing of children in school dormitories, in markets and places of worship," Buhari said.

"They have nothing to do with religion. They are terrorists and we are going to deal with them as we deal with terrorists."

Buhari, who recently became president after defeating former leader Goodluck Jonathan in March's general election, vowed to defeat the insurgents, who have caused thousands of deaths since their uprising started in 2009. The terrorists recently pledged allegiance to Islamic State (Isis) terror group.

The president also promised his country would do everything to bring some 220 missing schoolgirls, who have become known as the Chibok girls, home. They were kidnapped by the terrorists last April, amid reports they are being raped, forced to marry their abductors and used as suicide bombers.