More than a dozen soldiers died on Monday during a "major attack" by "terrorist armed groups" in northern Burkina Faso, the army said, adding that it could be the deadliest ever against the armed forces.
With other soldiers still missing, the death toll could pass 20, several security sources said.
"In the early morning, the military detachment of the Koutougou department in Soum province was the target of a major attack by armed terrorist groups," said a statement from the general staff.
"A provisional report states that more than a dozen soldiers were killed, and several were wounded."
The assailants used heavy weapons and burnt a large portion of the camp and material, a security source told AFP.
"In response to this barbaric attack, a large air and ground operation led to the neutralisation of several assailants," the general staff said without elaborating.
The poor Sahel state of Burkina Faso has been battling a rising wave of jihadist violence over the last four years which began in the north but has since spread to the east, near the border with Togo and Benin.
The heaviest Islamist attack against Burkina's army to date left 12 soldiers dead at Nassoumbou, also in Soum province, in December 2016.
More than 40 jihadists aboard pickup trucks and on motorcycles laid assault to a military post close to the Mali border.
Overnight Thursday to Friday armed men described as jihadists raided a village in the restive north, killing 15 people, plundering and burning shops, a regional governor said.
Most attacks in the former French colony are attributed to the Ansarul Islam group, which emerged near the Mali border in December 2016, and to the JNIM (Group to Support Islam and Muslims), which has sworn allegiance to Al-Qaeda in the Islamic Maghreb.
Those groups are believed to be responsible for around 500 deaths since 2015. Burkina's capital Ouagadougou has been attacked three times.
Former colonial ruler France has deployed 4,500 troops in Mali, Burkina Faso, Niger and Chad in a mission codenamed Barkhane to help local forces flush out jihadists.
Burkina Faso has also joined four other Sahel nations (Chad, Mali, Mauritania and Niger) in an initiative aimed at creating a joint 5,000-troop anti-terror force, also backed by France.