Niger, Mali and Burkina Faso say they will form a joint force to fight jihadist threats in their countries.
Niger’s army chief Moussa Salaou Barmou announced it would be operational as soon as possible, without giving details of its size.
He spoke in a televised speech on Wednesday following talks in the capital Niamey.
Groups linked to both Islamic State and al-Qaeda have killed thousands of people in the region in the past year.
The military regimes in the three countries have become increasingly close allies in recent months.
Last September, they formed a mutual defence pact known as the Alliance of Sahel States (AES), withdrawing from an international force, G5, that was set up to fight Islamists in the region.
Violence in West Africa’s Sahel region has worsened in recent years despite the military governments’ promises to deal with the decade-long conflict with jihadist groups.
The three countries have all severed their ties with France, the former colonial power, which for years had a strong military presence across the Sahel.
An UN peacekeeping mission in Mali, Minusma, that had been in place for over a decade was also ordered by the country’s leaders to withdraw “without delay”, and completed the exit in December.
The military regimes have strengthened ties with Russia, which has moved in to fill the void.
They have also announced they were leaving the West African economic bloc Ecowas.
They had already been suspended from the bloc, which has been urging the military regimes to ensure a quick return to democratic rule and imposed travel and economic sanctions on them.
Some of the sanctions were last month lifted or eased as Ecowas pushed for dialogue.
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