Niger and Chad on Monday attacked Boko Haram militants on islands on Lake Chad which the Islamists have used as a safe haven, military sources in Niger said.

A push by Nigeria's armed forces and troops from neighbouring Chad and Niger has driven Boko Haram from most of the positions it controlled earlier this year, reversing militant gains that forced Nigeria to delay elections slated for February.

"The islands are not easy to access but we have located the enemy positions. The air forces are bombing them right now," a Niger soldier based in regional capital Diffa told Reuters.

Another military source said a number of "terrorists" had been killed in the attacks, which saw troops dispatched in speedboats as well as air strikes, but gave no details.

"We are going to clear all the islands on Lake Chad. Boko Haram has chosen to seek refuge in certain hidden corners, but we will dislodge and wipe them out," he said.

There was no official comment from Niger or Chad. Nigerian forces were not involved, the military sources said. There was no comment from Nigeria on the operation.

A Chadian officer said about 50 Boko Haram militants had died alongside two Chadian soldiers in clashes over the nearby town of Abalam on Sunday.

The next target of Chadian and Niger forces is expected to be Malam Fatori, a town on Nigeria's border with Niger.

The offensive by Nigerian and regional forces has pushed Boko Haram, which seeks to carve out a caliphate in northeastern Nigeria, from all but three of the 20 local government areas occupied by the group at the beginning of the year.

Cameroon approved the establishment of a French liaison detachment to its forces based at Maroua in the Extreme North region who are fighting Boko Haram as part of the regional force, a statement from government spokesman Issa Tchiroma Bakary said.

Nigeria's rearranged election was held on Saturday, with millions voting despite pockets of violence that saw militants kill at least a dozen people. Coordination between regional forces has been tricky, however, with Chad and Niger complaining that Nigerian forces have been slow to deploy to take control of areas they have chased the militants out of.

Analysts say Boko Haram fighters are likely to revert to a campaign of hit-and-run attacks now they have been driven from the main towns they once controlled.