Seventy-one Chadian soldiers have been killed and 416 injured in two-and-a-half months' fighting Boko Haram, the Chadian army said on Friday.

Chad is taking part in a regional offensive against the Nigeria-based Islamist radicals along with troops from Nigeria, Niger and Cameroon.

Chad army chief General Brahim Seid told reporters in N'Djamena the "valiant soldiers" had died since February 3 in the "just and noble cause of bringing peace and security" to the region.

The countries neighbouring Nigeria, including Chad, sent troops to help crush Boko Haram after the Islamists began to threaten the region surrounding Lake Chad, where the borders of all four nations converge.

Nigeria's army has been criticised for failing to deal with Boko Haram fighters whose bid to set up a hardline Islamic state has killed more than 15,000 people since 2009 and put scores of thousands to flight, according to the UN.

The UN on Thursday launched an appeal for $174 million to give "life-saving aid" to the almost 200,000 Nigerians who have fled the country due to the group's brutal attacks.