At least 100 jihadist fighters have been killed in an operation against Boko Haram Islamists holed up on the islands of Lake Chad, the Chadian army said in a statement Friday.

"117 terrorists have been killed, two Chadian soldiers died and two others were wounded" in the operation against the Nigeria-based movement, which began two weeks ago, army spokesman Colonel Azem Bermendoa Agouna said.

"For two weeks, Boko Haram terrorists have been trying to infiltrate our islands on Lake Chad to varry out attacks on peaceful citizens," Azem said, adding that military operations were ongoing.

Lake Chad is a strategic region where the borders of Chad and northeast Nigeria converge with those of Cameroon and Niger, further north.

Boko Haram has carried out raids and suicide bombings in all four countries, which have formed a military alliance along with Benin to tackle the movement.

"Our armed forces and security forces have launched a vast offensive to dig out and neutralise these terrorists on these islands," Azem said.

No immediate confirmation of the death toll could be had from independent sources.

"Several boats have been destroyed and several weapons of different calibres have been recovered," added the colonel, who specified that the operation had notably targeted the island villages of Koungya, Merikouta, Choua and Blarigui.

"The sweep continues and the definitive toll will be released later," the spokesman said.

About 1,000 Chadian soldiers have been deployed in the area of Lake Chad "to occupy all the islands and neutralise Boko Haram", a security source said. Chad's government urged local villagers to leave before the offensive started.

Boko Haram forces, who have killed some 15,000 people since their insurgency began in 2009, use the Lake Chad islands to fall back after coming under heavy attack in Nigeria by the regional coalition, in which the Chadian army has taken a leading role since the joint operation was formed early this year.