Authorities in Burkina Faso have declared a two-day period of mourning after suspected militants killed at least 41 members of a government-backed militia in the country's desert north this week.

A column of fighters from the Homeland Defense Volunteers (VDP), a group the government funds and trains to contain Islamist insurgents, was ambushed on Thursday as it swept a remote area in the northern Loroum Province, authorities said on Saturday.