Four people including two Spanish citizens and an Irish national are missing after an armed ambush on an anti-poaching patrol in eastern Burkina Faso on Monday.
No one has claimed responsibility for the attack.
The attackers struck during the day on a road leading to the vast forested reserve of Pama. Those missing include a member of the Burkinabe armed forces, two Spanish citizens and an Irish national who worked for conservation organisations.
There was no immediate comment from the Burkina Faso government.
An official at the Spanish foreign ministry confirmed that two Spanish citizens are missing in Burkina Faso.
"We are in close contact with the families and we keep them informed punctually about everything that is happening and the efforts carried out to locate the two Spaniards," the official said, adding that Spain was also in touch with Burkina Faso authorities to help locate the missing people.
Ireland's foreign ministry said late on Monday it was aware of the reports and is liaising closely with international partners.
The worsening violence in Burkina Faso has led to one of the world's fastest-growing displacement crises, the United Nations warned earlier in April. The Sahel now hosts nearly three million refugees and people displaced inside their own country.
Insurgents are believed to be holding a number of foreign hostages in Burkina Faso, Mali and Niger.
Burkina Faso, like much of West Africa's Sahel region, faces a deepening security crisis as groups with links to al Qaeda and IS carry out attacks on the army and civilians, despite help from French and UN forces.