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That toll does not include an unknown number of the security forces who may have been killed in the attack. The toll was likely to rise as there are a large number of people still unaccounted for, according to a security source.

Two security sources said the military vehicle leading the convoy was struck by an IED on a stretch of road where there is no cellphone network.

Shortly after the initial explosion, an unknown number of gunmen opened fire. One of the sources said it appeared that they targeted the buses as well as the military escort, which was unusual.

In December a police vehicle was attacked on the same road, resulting in five deaths.

When contacted by Reuters, a Semafo spokeswoman said: “At this point, we do not have full information and are not in a position to add to this morning’s release.”

The Boungou mine is secure and operations are not affected, the company said.

Semafo’s Toronto-listed shares hit a nine-month low after the news and were trading down 10% by 1755 GMT.

Once a pocket of relative calm in the Sahel, Burkina has suffered a homegrown insurgency for the past three years, which has been amplified by a spillover of jihadist violence and criminality from its chaotic northern neighbor Mali.

After last year’s incidents, which Semafo said were the work of “armed bandits,” the company reinforced its escorts and decided to transport all expatriate employees by helicopter between the Boungou mine and Ouagadougou. (Reporting by David Lewis, Helen Reid and Zandi Shabalala Writing by Helen Reid and Alessandra Prentice Editing by Catherine Evans, Alexandra Hudson and Leslie Adler)