President Marc Christian Kabore said the mourning period, which runs until midnight Sunday, is to be held in memory of the security forces who "fell following a terrorist attack" on Monday against the military base in Koutougou, in Soum province near the border with Mali.

A security source told AFP this week that the attack, which also wounded seven soldiers, appeared to be "well-prepared and coordinated", with several dozen militants who arrived on motorbikes and pickup trucks and fired heavy weapons including rockets.

The period of mourning will include flags flown at half mast on all public buildings, said the decree, which forbade public "celebrations (and) recreational events".

A former French colony that ranks among one of the poorest countries in the world, Burkina Faso has been struggling with an Islamist revolt since 2015, which began in the north but has since spread to the east.

Five soldiers initially thought to be missing after Monday's attack were found alive, according to the armed forces general staff.

Previously, the heaviest Islamist attack against Burkina's army left 12 soldiers dead at Nassoumbou, also in Soum province, in December 2016.

Most of the attacks have been attributed to the Ansarul Islam group, which emerged near the Mali border in December 2016, and to the Group to Support Islam and Muslims (GSIM), which has sworn allegiance to Al-Qaeda in the Islamic Maghreb (AQIM).

Those groups are believed to have been responsible for around 500 deaths. Burkina's capital Ouagadougou has been attacked three times.